A precipitation-pyrolysis process was used for the synthesis of cubic nano-MgO powder. The main effect factors on the size and shape of nano-MgO powder such as mechanical stirring rate, organic reagent, and calcinations of the process were investigated. XRD and TEM were used to study on composition, morphology and size of nano-MgO powder. The results indicate that at the condition of mechanical stirring rate 510 rmp, glycol as organic crystal controller, calcination time 3 h, cubic nano-MgO powder was prepared with average size of 35 nm. FTIR results show that cubic nano-MgO powder has abnormal infrared absorption behaviors compared with that of micron scale.