Thanks to the rapid development of variable production patterns of industries, many kinds of chemicals are used. But the chemicals usually come with dangers of causing fires, explosions, and harming people. In the past decade in Taiwan, these chemicals caused many serious industrial disasters. They happened not only in conventional industries but also in semiconductor and chemical industries. And most of them happened due to the leaking of flammable or toxic gases. In the situation of the outbreak of fires, explosions may occur, and they will generally bring about heat radiation, explosive pressure, and energy releasing. And all of these often harm the workers and the environment, and also bring great loss for the factories. In order to prevent the disasters, except for improving the protection and safety equipment, it¡¦s more important to realize how to use effective ways to reduce them. If the gas pipes pass through a densely populated area, when some toxic gas leaks, it will cause fatal dangers which result from the mixing of gas diffusion and air, the flow process of the mixture, concentration of CH4, and temperature distribution, or explosions. After that, some toxic gas with poisonous substances will be released, and it will turn out to be horrible consequences that are beyond our imagination. So it¡¦s really important to do research on gas leaking and gas diffusion. If gas-leaking simulation is applied on the analysis of the leaking of flammable gas and gas diffusion, it¡¦s much more possible to protect the workers from being hurt, keep public safety, and reduce the loss on the wealth of the society. The thesis focuses on building various hazard patterns of gas leaking, gas explosions, and chemicals, etc. From all of these, the initial conditions and the degrees of dangers will be revealed. In the thesis, numerical simulation is used to analyze the density, pressure, speed of the leaking gas and all the distributions in the flow field. The major analysis is about the effects of parameter and the display of concentration distribution, and hazard range.