When systems or processes are complicated, the major cause which affects the operation is usually not easy to find. Engineers take advantage of the viewpoint of probability to analyze the characteristics of systems. Reliability models can describe the failure behavior of systems in the particular and continuous form. Hence the observation of characteristics of the reliability models is the main point in this paper through the variation of parameters in particular hazard rate function. It mentions that probability density function is related to the hazard rate function in the definition of reliability. We can understand the trend of different distribution by analyzing the characteristics of probability density function. It is significant to research the features of particular reliability models by the discussion of probability density function.