A crystalline form of 3-(2-chloro-1,3-thiazol-5-ylmethyl)-5-methyl-1,3,5-oxadiazinan-4-yledene (nitro)amine (thiamethoxam) is provided. The crystalline form exhibits an X-ray powder diffraction pattern having characteristic peaks (expressed in degrees 2θ +/- 0.2° θ) at one or more of the following positions: 6.09, 15.37, 17.83, 18.43, 20.86, 22.01, 26.95 and 27.84, and an Infrared (IR) spectrum having characteristic peaks at about 2933.62, 2161.78 and 1593.88 cm-1. A method of preparing the crystalline form comprises crystallizing 3-(2-chloro-1,3-thiazol-5-ylmethyl)-5-methyl-1,3,5-oxadiazinan -4-yledene (nitro)amine (thiamethoxam) from a solvent system comprising a solvent selected from an alcohol, a glycol, an ether, a ketone, an ester, an amide, a nitrile, an aliphatic or aromatic hydrocarbon, or mixtures thereofl and isolating the resulting crystals.提供一種3-(2-氯-1,3-噻唑-5-基甲基)-5-甲基-1,3,5-噁二嗪-4-亞基(硝基)胺(噻蟲嗪)的晶形。該晶形的X-射線粉末衍射圖在以下一個或多個位置時出現特徵峰(2θ誤差範圍為+/- 0.2° θ)：6.09、15.37、17.83、18.43、20.86、22.01、26.95及27.84，以及紅外線光譜大概在2933.62、2161.78及1593.88cm-1時出現特徵峰。一種製備該晶形的方法，該方法包括從溶劑系統中形成3-(2-氯-1,3-噻唑-5-基甲基)-5-甲基-1,3,5-噁二嗪-4-亞基(硝基)胺(噻蟲嗪)的結晶，該溶劑系統包含選自醇、二醇、醚、酮、酯、醯胺、腈、脂族或芳族烴，或其混合物之溶劑；再將得到的結晶單離。
