In this paper, we studied the temperature dependency effect of thermal coefficient of resistance (TCR) in amorphous silicon (a-Si) on the properties of uncooled microbolometer with a-Si as a resistance layer by simulation. The temperature of the microbolometer rises during the operation mainly due to the heat generated by Joule heating as well as IR radiation. Generally, the TCR of a-Si is treated as a constant for the simplicity but the absolute value of TCR has been reported to decrease as the temperature increases. Therefore, to improve the device characteristics, the effect of temperature dependency of TCR in a-Si should be considered carefully in the range of the operating temperature. The responsivities of microbolometer are simulated according to the width of the resistance layer (W) with TCR as a function of temperature, which shows that the optimal W condition is affected by the TCR value changed by the temperature.