Spherical NiO nanocrystals were obtained by thermal decomposition of the precursor obtained via a hydrothermal method using urea as precipitant and polyethylene glycol (PEG) as surfactant. The structure, morphology and magnetic properties of the products were examined by XRD, TEM, ED, IR and VSM. The results of the structure and magnetic measurements on NiO nanocrystals were discussed. The results showed that the products were nanocrystalline NiO with a diameter of 21 and 50 nm, respectively, after calcined at 300 and 500 oC. The calcined NiO nanocrystals exhibited the characteristics of weak ferromagnetism by magnetic analysis at room temperature. At low external field, the hysteresis loops exhibit low coercivity, Hc=144.7 and 200.5 Oe, for the samples calcined at 300 and 500 oC, respectively.