Optical flow is widely used in image and video processing. This paper describes the definition of optical flow and some basic methods of estimating optical flow. There are three main optical flow methods described in this paper: Horn–Schunck method, Lucas–Kanade method and Anandan method. It will select the most appropriate method to measure the flow in videos containing smoke and gas by comparing those three methods with different criteria. The results are as below 1) Horn–Schunck method is good at measuring flow in infrared video and in horizontal direction, when 2) Anandan method is adept in estimating upward optical flow but not suitable for infrared video. 3) Lucas-Kanade method can be used not only in smoke videos, but also in methane gas videos, and it can detect flow in both horizontal and vertical direction.