As a mature flow field measurement technology, particle image velocimetry (PIV) has been widely used to calculate the instantaneous motion of particles by identifying and comparing the positions of tracer particles between successive image frames. For example, the ambient air velocity distributions around fuel spray can be measured using LIF-PIV However, due to the need to accurately identify the tracer particles, PIV is limited in its applications, such as measuring the flow field in turbid media. Optical flow calculates apparent velocities of movement of brightness patterns in an image. Different from PIV, optical flow segments images into regions that corresponds to different objects without requirement of tracer particles. Since it is based on brightness patterns, optical flow may be widely used in measuring flow field in the area of fuel spray and in-cylinder combustion. To check the accuracy and robustness of optical flow, this study will calculate the 2-dimensional velocity fields of the same PIV image sequences using iterative optical flow and PIV respectively. Using PIV results as criterions, precision and accuracy of optical flow are studied.