ZnS:Mn²⁺ nanocrystals (NCs) with particle size from 1.9 nm to 3.2 nm are synthesized via chemical precipitation method with different [S^2-]/[Zn²⁺] ratios. The size??dependent decay for Mn emission exhibits a double exponential behavior. And two lifetime values, in millisecond time domain, can both be shortened with size increasing, which is attributed to enhanced interaction between host and Mn²⁺ impurity. A molecular structure model is proposed to interpret the tendency of two lifetime components, which is correlated to the number of S vacancy (V_s) defects around Mn²⁺.