Silica nanospheres were prepared in polyether oil with the insitu solgel method.Effects of the ammonia concentration,water amount,tetraethyl orthosilicate (TEOS) concentration,polyether oil loading and reaction temperature on the size and morphology of SiO2 nanoparticles were investigated by transmission electron microscopy (TEM).The formation mechanism of the resultant SiO2 particles under different reaction conditions was discussed.An experimental study on friction reducing and wearresistance performances of nanosilica lubricating fluid with different nanoparticle concentration and size were carried out by utilizing fourball friction and wear testing machine.The results show that the friction coefficient and wear scar diameter of the nanosilica lubricating fluid is firstly decreased then increased with the increasing of nanoparticle concentration,and at the optimum concentration of nanoparticles,the effect of friction reducing and wearresistance is enhanced as the nanoparticles diameter decreases.The lubricating fluid shows better friction reducing and antiwear performance,however,due to the size variation,the optimum concentration did not show irregularity change.