In this paper, a series of p-Chloromethyl styrene(p-CMS)/Styrene(St) miniemulsion was prepared with the ultrasonication method. Effect of amount of emulsifier, ultrasonic time, ultrasonic power, and costabilizer content on preparation of miniemulsion and effect of p-CMS/St mass ratios on its stability were investigated. The results show that a miniemulsion with an average diameter as low as 99 nm and a polydispersity index of 0.12 was achieved, under the condition that the concentration of sodium dodecyl sulfate is 10 mg/mL, ultrasonic time is 24min, ultrasonic power is 400W and the mass ratio of hexadecane to monomer is 5%. The stability of the miniemulsion decrease with incerase the mass ratio of p-CMS/St, and Ostwald ripening may be the prime reason for the instability of p-CMS/St miniemulsion. ratio of p-CMS/St, and Ostwald ripening may be the prime reason for the instability of p-CMS/St miniemulsion.