This report describes the reaction of N-(thio)phosphoryl imines with diethylzinc under different conditions. An interesting and distinct chemoselectivity between hydrogen-addition and ethyl-addition to imine double bond is disclosed: in weakly polar solvent, e.g. toluene, N-(thio)phosphoryl imines were exclusively reduced in excellent yields via a β-H transfer from diethylzinc to imine double bond; in polar solvents like THF, the reduction product was competitively formed as a major product together with the minor product resulting from ethyl-addition to imine double bond; in sharp contrast, in the presence of strong coordinative additive N,N,N0,N0-tetramethylethylenediamine (TMEDA), the ethylation product was formed exclusively from the reaction of N-(thio)phosphoryl imine with diethylzinc. These results are discussed and explained in terms of the coordination interactions between the imine, solvent, and additive with diethylzinc.