The paper described the development of a new three-phase power monitor for the need of national smart grid.The instrument used LPC2478 ARM processor with external 8-channel synchronous sampling ADC,and standard RS485 interface.The measuring accuracy of main parameters was better than 0.2 grade,and passed the tests and certification of Electric Power Research Institute of Jiangsu province.The overall test performance achieved A-level of national standard for power quality.