In photoelastic stress analysis, the accuracy of analysis can be improved by using fractional isochromatic fringes, so it is very important to study the approach for producing fractional fringes. By using image processing technology, a fringe pattern is captured firstly, then fractional fringes can be produced by arithmetic operation to the fringe pattern. Theoretical analysis is given in detail, and experiments are carried out. Seven fractional fringes are produced between two integer fringes by using the technique.