The conversion efficiency of heat to electricity is the basic parameter of thermoelectric element, thermoelectric unicouple and thermoelectric devices. In principle, the heat to electricity conversion efficiency of thermoelectric element has been defined as the electrical output power of the element divided by its thermal input power. Due to the heat loss by convection and radiation heat transfer the test result of the heat to electricity conversion efficiency has a large errors. The authors present a test method for heat to electricity conversion efficiency of thermoelectric unicouple. The thermal input power of thermoelectric unicouple has been divided into the electrical output power plus thermal output power out of the cold end of the unicouple. The later has been determined by a thermoelectric thermal power meter. The method avoids the difficulties to measure the input thermal power into the hot side of the unicouple, so that the convection and radiation heat lose out of the unicouple side can be ignored. Owing to Seebeck Coefficient of the thermoelectric semiconductor materials could be many times of the metals, the thermoelectric thermal power meter has high sensitivity, so that high test precision could be gained in test for conversion efficiency of thermoelectric unicouple. The paper presents some test results for heat to electricity conversion efficiency of thermoelectric unicouple, and discusses about the factors which affect the test results.