Abstract: Several siable polyether-Oil-based ferrofluids were prepared by dispersing magnetic particles in polyether oil with a polyether-phosphate surfactant.The magnetic particle was made by co-precipitating the Fe²⁺,Fe³⁺,Fe³⁺ salt solutions with a base,Examined by TEM,the magnetite with an average diameter about 10nm was dispersed very well.A sample ferrofluid RE-Ⅲ (4πM_s=0.0450 T) had been used in the rotatory seal in an X-ray diffraction instrument,which has been working at 3000r/min and 2.7×10^-5 pa for more than two years.

Key words: ferrofluid, polyether, rotatory seal