Magneto-rheological fluid (MRF) is non-colloidal suspension. In this paper, it is introduced that MRF will produce magnetic coagulation at an applied magnetic field by analyzing the interact energy between particles in MRF, and the coagulation is chain-like flocculation. The main reason that MRF produced magnetic coagulation is the direction between chain-like structure and magnetic field is the same. According to the studies above, the formula of yield stress for MRF at an applied magnetic field can be deduced. It can be seen from the formula that there is a square relationship between yield stress of MRF and particle magnetization, and the yield stress of MRF is closely related with volume concentration. The conclusions above match the results of existing experiments very well.