Double yield points before necking : were observed in injection molded specimens of poly(tetramethylene terephthalate) (PTMT) and its copolymers under tensile loading. The first yield is associated with deformation of the amorphous region and the second yield is caused by the alpha to beta transition of PTMT crystallites. The first yield point became less apparent with an increase of crystallinity of the specimens. The second yield point became more apparent with an increase of crystallinity. Annealing of injection molded specimens increased the crystallinity and increased the second yield point on stress-strain curves. Copolymerization decreased the crystallinity and made the first yield point more prominent. Effects of annealing on mechanical and thermal properties of the specimens were also measured. The specimens changed their properties from ductile to brittle during annealing. Their change during annealing was mainly attributed to the increase of crystallinity and not to thermal degradation and/or crosslinking.