Abstract: In order to detect and evaluate the flatness of small plane of diffuse reflection, a method was presented based on carrier fringe phase analysis. With a laser as illuminating light source, a coherent fringe pattern at period of 100m was achieved. The modulated phase distribution of the testing surface was retrieved after Fourier transform and converted into the physical height distribution, and then these height data were applied to evaluate the surface flatness. The detailed theoretical analysis was presented. The theoretical height resolution can reach 10m. Due to the effect of the noise, the experimental height resolution only reached 20m. The experimental results prove that the method is valid.