In this study, the dependence of Pd/GaAs Schottky diode on the electroless plating (EP) variable is systematically studied. Both alkaline and acidic formulas for electroless Pd depositions are employed for investigations. The correlation between Pd gain size and the manipulation of plating bath composition is constructed. Experimental results show that the performances of Pd/GaAs Schottky diode, electric rectification and hydrogen detection, are largely governed by the Pd grain size. Furthermore, without the interference of sodium ion, the acidic-plated Pd/GaAs diode with intermediate oxide layer exhibits the excellent hydrogen sensing performances from 15 ppm to 1.0 % H2/air.