The heat transfer characteristics of the spray cooling with ammonia are experimentally investigated on a smooth surface and two porous surfaces with different sintered grain sizes. It is found that porous surfaces significantly enhance the heat transfer as a result of wicking action of porous structure and potential active nucleation sites. In nucleate boiling region, with the decrease of sintered grain size, heat transfer of porous surfaces performs better; the maximum heat transfer coefficients of 100612 W/(m2·K) and 96464 W/(m2·K) are achieved with the heat flux of 367 W/cm2 at the flow rate of 0.0133 m3 /(m2·s) for the porous surfaces with sintered grain sizes of 28 μm and 49 μm. When the flow rate increases from 0.0133 m3/(m2·s) to 0.0181 m3/(m2·s), the appearance of critical heat flux (CHF) density is delayed and the heat transfer coefficient is increased by 47% to reach 147503 W/(m2·K) at the heat flux of 367 W/cm2.