A precipitation-pyrolysis process was developed for the synthesis of MgO nano-particles with diameter of 20~100 nm by using organic reagent, glycol, n-butanol and N,N-Dimethylacetamide, as shape and size controller. XRD and TEM were used to study on composition, morphology and size of the as prepared MgO nanoparticles. The results indicate that organic crystal controller is one of the key factors affecting the sizes and shapes of the nano MgO particles. When using glycol as a crystal controller, the dispersion of nano MgO particles is better and olive-shaped particles with sizes of 20~40 nm are prepared. When nano MgO particles are synthesized by MgCl2 and Mg(NO3)2, their particle sizes are smaller and their particles with olive shape. During chemical reaction process, Mg(OH)2 particles are already formed to be nano scale. Calcination process at the temperature of 650°C can change the crystal shape slightly. FTIR results show that nano MgO particles have abnormal infrared absorption behaviors compared with that of micron scale.