Abstract: The single oocyst separation, propagation and precocious strain selection were conducted in a strain of chicken Eimeria mitis designated as SY-MB which was isolated from Bayannur region of Inner Monlolia according to the methods established by Jeffers in 1975. After 15 successive generations inoculation, the latent period of the isolated strain was 19 hours shorted （from 96 hours to 77 hours）. The selected precocious strain designated as SY-mb77 was unselectively sub-cultured for 10 generations by the methods reported by Sunxun and others in 1998. The latent period, oocyst production, immunogenicity and pathogenicity of the selected strain possessed the characteristic of a typical precocious strain. The precocious strain can effectively protect the inoculated chickens from invasion of high virulent strain.