Abstract: Influences of composition, heterogenety, temperature, endothermal-reaction, exothermal-reaction in chemical etchants on the uniformity of the lapped GaAs wafer are investigated. Based on the investigation, ammonia series are proposed to use as the etchant and the wafer uniformity of 1%1.47% is achieved, and the GaAs wafer with the better flatness can be provided for the next process.