Wireless communication systems are increasingly deployed by family, businesses, government and SOHO users because of the freedom wireless communications afford, simplicity of installation and ease of use. Along with its rapid development, the security problem in wireless communication network has attracted more and more attention. It has been estimated that the IEEE 80.211 standard is susceptible to attacks both on data content and user authentication, such as eavesdropping, spoofing and Denial of Service. These attacks are due to the standard's inherent flaws and space signal sharing caused by the omni-directional antenna it usually uses. In recent years, few studies have been undertaken which examine the use of the smart directional antenna to mitigate the security risks in mobile wireless computing networks. One of the major reasons is the antenna size, as portability is a key requirement. Due to the dielectric-material embedding and FDTD optimal design technology, a smart antenna with controllable directionality can now achieve an overall volume reduction of 80%. This has promise for application in the wireless security realm. This thesis provides a conventional background in wireless computing networks and security theory. In addition, security level definition, program in Matlab™ and corresponding experimental results are provided. To show the effectiveness of improving security in WLAN with the use of a smart directional antenna, this thesis proposes a model for predicting electromagnetic environments using a directional transmission antenna based on the Finite Element method. To validate the numerical results, a new experimental method (using Network Stumbler software) to measure the signal strength in different locations is introduced. Results from experiments using the two methods are consistent. The latter method gives the users the possibility of using a laptop (with an appropriate wireless card, software and roguing around) to detect the real-time signal strength in different locations instead of using expensive measure apparatus. Such an advantage can be valuable for the efficient implementation of indoor wireless networks for security purposes. Finally, it contributes a security solution using DE-ESMB and DE-ESPAR smart antennas in WLAN. Through these simulated and experimental results, one can conclude that the security performance in WLAN can be greatly improved by using smart directional antenna. There are five chapters in this thesis: Chapter 1 introduces the wireless communication network. Information about architecture, topology and popular wireless standards is presented in this chapter. Chapter 2 investigates the security problem in IEEE 802.11 Standards. Recent improvements in security, and their limitations, are also included. This problem is investigated by performing laboratory experiments, including a real attack on 802.11 Standards and packet capture experiments using AiroPeek NX software. Chapter 3 describes the useful parameters of smart directional antennas, and then presents the security level definition in WLAN. Simple software in Matlab™ to determine the security level according to the beamwidth of the adopted antenna is also introduced. Chapter 4 is the important part of this thesis. Simulation results of signal strength in different locations using FEMLAB™ are presented, which are consistent with the follow-up experimental results carried out in a typical office area. This chapter also describes the final security solution using the newly-designed DE-ESMB and DE-ESPAR directional antennas. Chapter 5 is a summary of the thesis. It presents the conclusions and suggests area of future research for improving the security level using smart antennas in WLAN.