Benzoyl styrene derivatives were synthesized by Claisen-Schmidt condensation. It was measured that nucleophilic addition of thiourea for benzoyl styrene made use of ultraviolet spectrophotometery at a wide pH 1.0~13.0 range in 5% dioxane-H2O at 40℃. On the basis of general base catalysis, substitutent effect, and confirmation of addtion reaction product, the nucleophilic addtion kinetics of thiourea for benzoylstyrene derivatives were measured by pH change. It maybe concluded that a part was unrelated to pH and another part was in proportion to concentration of hydroxide ion : Above pH 10.0. It was in propotion to concentration of hydroxide ion, a part having no concern with pH was added to the neutral thiourea molecule. From the results of measurement the reaction rate and there findings, nucleophilic addition of thiourea to benzoylstyrene derivative was proposed a fitting mechanism.