The impacts of boron (B) through root application and foliar spray on arsenic (As) uptake and efflux by/in rice seedlings (Oryza sativa L.) were investigated in three hydroponic experiments. Addition of B in culture medium didn’t alter concentrations of arsenite [As(III)], arsenate [As(V)] and total As in rice seedlings under either As(III) or As(V) exposure. Foliar B supply decreased root As concentrations by 20.9% under As(V) treatment and by 12.6% under As(III) treatment, respectively, yet didn’t significantly decrease shoot As concentrations. Concentrations of As(V) were positively related to B concentration in rice root under As(V) treatment following foliar B supply (P < 0.05). Rice seedlings extruded 105.2% more As after As(III)-pretreatment than after As(V)-pretreatment. Foliar B supply increased the amount of As excreted by As(III)-pretreated rice root by 14.0%–16.9% (P > 0.05) and made no effect on As efflux for As(V)-pretreatment seedlings. These results indicate that root application of B at four times the concentration of As can hardly decrease As accumulation by rice, whereas foliar B supply is conducive to a decline in As acquisition by rice root. It is likely that B channel is at least not the main pathway for As(III) entering into rice root, and As(V) distribution mechanism in rice plant may share with B.