The all optical integrator based on modal dispersion is studied in thi s paper through numerical simulation and experiment. The numerical simulation shows that the time delay induced by modal dispersion i n step-index multimode fiber (MMF) is close to linear.A cost effective method uses a tilted input optical beam in one dim ension which is launched into the MMF to excite different modes.Time delay is generated between different modes by modal dispersion as th e optical signals are transmitted through the MMF,and the optical signal in different modes can overlap with each other.A large area h igh speed photodiode (PD) is used to receive the optical signal in different modes.An all optic al integrator with time window longer than 1.5ns is achieved in an MMF with length of 50m.An applicat ion of the all optical in tegrator is demonstrated.Since the system is mainly based on modal dispersion,it′s independent of the wavelength of the opti cal sources.Its time window is much larger than that of the system based on chromatic dispersion using the same length fiber.Besides that,because of the one dimension stretching before the optical beam launched into the MMF,it has good flexibility .