The effect of the magnetic field on Shapiro steps is studied in YBa2Cu3Oy, step-edge junctions. Theoretically, we show the predictions of the voltage source model and the current source model for Shapiro steps modulated by magnetic flux. Experimentally, the magnetic field dependence of the height of Shapiro steps was measured at a fixed microwave power. Experimental results are qualitatively consistent with the predictions of the current source model.