A five-membered phosphoroheterocycle, 5, 5-dimethyl-2-(4′-methoxy)phenyl-1, 3, 2-thiazaphospholidine-4-thione 2-sulfide, has been prepared by the reaction of Lawesson's reagent (LR) with α-hydroxyl isobutyronitrile, and its crystal structure was determined by X-ray diffraction method. The title compound (C11H14NOPS3) is triclinic with space group P ī, a= 6.8418 (9), b= 10.5467 (1), c= 11.5670 (2)A, α= 114.851 (2),β= 100.291 (2),γ= 91.390 (3)o, V= 740.7 (2) A3, Z= 2, Dc= 1.360 g/cm3, λ= 0.71073 A, μ(MoKα)= 0.59 mm-1, F(000)= 316. The Structure was refined to R= 0.0476, wR= 0.1272 for 2549 unique reflections with I >2σ(I). It contains a phenyl ring and a 5-membered phosphoroheterocycle with the dihedral angle of 87.06 (9)°. The five atoms (P (1), N (1), C (1), C (2) and S (2)) of the heterocycle are nearly coplanar. In view of the unit cell it can be concluded that there exists an intermolecular hydrogen bond in the form of Sa┈H-N (a:-X,-Y+1,-Z+1). The existence of d-pлbond between P (1) and N (1) was found as well as unsaturated property of N (1)-C (1) moiety.