A new method is presented to extract quantitative information of Moire fringes. It is different from the method of phase shift. Instead of changing the phase of Moire fringe, it changes the frequency of the Moire fringe. The theoretical formula is introduced for a special relationship of three frequencies. For unwrapping the phase image, the first step is to expend the cosine value range from (0, π) to (0, 2π) to avoid the indetermination of “plus” or “minus”. The experimental result is given and the effect of light source position on measuring results are discussed.