Abstract: It is shown that a Fabry Perot-interferometer with small wedge angle or a Fizeau interferometer is illuminated by a collimated multiple longitudinal mode He-Ne laser beam,the spacing of adjacent fringes depends on the ratio r=d/L ,here d and L are the length of the interferometer and laser cavity.When r is a integral,the different order fringes corresponding to different modes are overlapped and the intensity of fringe is maximum and the fringe spacing represents λ/2.When r is fraction,r=N/M,N and M are both prime numbers,the fringe spacing is corresponding to λ/M.It is easy to get M= 10 for a long cavity He-Ne laser,the interferometric sensitivity is then 10 times that of M-Z interferometer.This interferometer can be applied to examining of the optical components with high precision and the measurement of low flow field.Some applications are presented in this paper.