The fluid magnetic abrasives (FMA) are a new type of precision finishing abrasives, which can be used to finish the parts with intricate or complex shape to a quite low surface roughness value. As a key parameter, the magnetic saturation flux density has a great impact on the design of finishing devices and the control of finishing process. In order to measure the magnetic flux density and determine the magnetization curve of FMA, a new apparatus has been developed. The measuring mechanism and the experimental apparatus are presented. The experimental results are discussed as well in this paper.