Abstract: Field observational previously indicated a mouth bar of a fan delta could exhibit a fining- or coarsening-upward trend, which bring a new challenge to the identification of mouth bar in subsurface studies due to the lack of morphological descriptions. Previous studies have indicated that effluent behavior in river-mouth system can affect the vertical grain-size trend of mouth bar, but the drivers and magnitude of this phenomenon are not understood. We conducted flume experiments to investigate the mechanism and controlling factors of vertical grain-size trend of mouth bar. Experiment with a steeper slope of the substrate layer, greater discharge, higher sediment/water ratio, and coarser sediment induced a fining-upward trend of mouth bar, because the effluent was dominated by strong inertia. Mouth bar in the experiment with a gentler slope of the substrate layer, smaller discharge, lower sediment/water ratio, and finer sediment exhibited a coarsening-upward trend dominated by the friction-dominated effluent. The relationship between the vertical grain-size trend of mouth bar and the gradients of foreset bedding in small-scale flume models and the cut-off of 15°–18° are applicable in natural systems. Identifying depositional setting to infer depositional process in river-mouth system and analyzing the plane geometry of sandbodies are two steps in the interpretation of ancient fan deltaic rock record.