An equimolar mixture of poly(L-lactide) and poly(D-lactide) was crystallized into a stereocomplex whose crystal system is triclinic (P1) with cell dimensions:a= 0.916 nm,b= 0.916 nra,c(chain axis) = 0.870 nm, α = 109.2°, β - 109.2°, and γ = 109.8°. In the unit cell, a poly(L-lactide) segment and a poly (D-lactide) segment are contained as a pair and packed laterally in parallel fashion. The L- and D-poly(lactides) in the complex take a 31helical conformation, which is a little extended from a 103helix in the homopolymer crystal with the α-form. Homopolymers are also able to take the 31helical conformation and form the β-form crystal. The 31helix in the homopolymer crystal is less stable than the 103one, and hence the β-form is easily transformed to the α-form by annealing.