In this thesis, the investigation focuses on the analysis of the high frequency characteristics and the nonlinearity of the transistors. In view of the III-V semiconductors which have excellent high frequency performance and the advantage for high frequency circuit design, the 0.15£gm InGaAs based pseudomorphic high electron mobility transistors provided by WIN semiconductor Corp. were used in this study. The high frequency measurement was utilized to extract both extrinsic and intrinsic components of the transistors, and further to establish the small signal equivalent model in each bias condition. According to the physical definition of the extracted gm, gds and the relationship with the output current, RF I-V curves could be determined through the integration procedure. The nonlinearity of the transistors can be attributed to the nonlinear input capacitance Cgs and Cgd, and the voltage dependent current source. The high frequency nonlinear models proposed in this thesis were based on classic Angelov model. For the high frequency application, the frequency dependent characteristics of the nonlinear sources would be taken into consideration through the combination of the RF I-V curves and extracted intrinsic components. Thus, the nonlinearities could be able to describe by nonlinear function through the fitting process and model the output performance completely. The accuracy of the models could be confirmed through the comparison between the simulation and the measurement result. Obviously, the high frequency models which include the high frequency effect and the nonlinear characteristics have excellent agreement with the experimental data.