Several samples of poly(lactic acid) with different molecular weights and tacticity have been prepared, and some PLLA injection moulded specimens have been annealed to promote their crystallization. From the characterization data, poly(L-lactide) showed more interesting mechanical properties than poly(D,L-lactide), and its behavior significantly improves with crystallization. In fact, annealed specimens possess higher values of tensional and flexural modulus of elasticity, Izod impact strength, and heat resistance. The plateau region of flexural strength as a function of molecular weights appears around M(upsilon) = 35,000 for PDLLA and amorphous PLLA and at higher molecular weight, around M(upsilon) = 55,000, for crystalline PLLA. The study of temperature effect shows that at 56 degrees C only crystalline PLLA still exhibits useful mechanical properties.