The effect of long period wave to local sand transport rate in the surf zone is analyzed by laboratory and numerical experiments. In the laboratory experiment, surface profile, near-bottom velocity and suspended sand concentration are measures simultaneously and resultant net sand transport rate is measured by mass difference during the experimental runs. It appears that the net sand transport rate is determined by three driving forces which are undertow, long wave and short wave. In mild slope condition, the effect of long wave becomes large. In order to simulate this process, a new numerical model is formulated.