The objectives of this research were focused on the effect of formulation on the stability and bioactivity of new 50% formulated solution. Laboratory studies on the storage stability under an accelerated condition (50°C), the physical performance test with 622 ppm hard water, and the 48 h mortality on Plutella xylostella L. were conducted. Results from the stability-analysis and physical performance test showed that about 16 new formulated 50% solutions were in acceptable quality as compared with the OECD (Organization for Economic Cooperation and Development) guideline, and the sixteen formulated products were then analyzed for the 48 h mortality on the Plutella xylostella L. at recommended rate in dipping method. The result indicated that formulas of 3, 8 and 9 were better than the selected domestic product and imported product in both stability and bioactivity.