Abstract: Microcapsules with an urea-formaldehyde resin wall were prepared for fipronil by an in situ condensation polymerization reaction in aqueous medium. Several reaction parameters were invest igated, including the acidification time, mixing rate, emulsion and solidify method on size, particle size distribution, and microcapsule structure. The results showed: Optimum reaction conditions were as follows: solidify time 90 min, mixing rate 3 000 r/min, acidification time 120 min with suitable emulsion 0201B. Under these conditions, solid, spherical microencapsules formed with uniform diameter and good covering. And so solid, spherical microencapsules with good flowability could be formed by the solidify of spray-drying technigue.