Two-beam polarizing interference (Pluta) microscope was used to study the effect of annealing conditions (temperature and duration) and strain rate on the physical properties of isotactic polypropylene (iPP) fibers. The percentage shrinkage of the fiber length at different annealing conditions was studied. The effect of strain rate on the birefringence and molecular orientation of stretched iPP fibers was carried out before and after annealing. The test samples of iPP fibers were treated at two annealing temperatures: 80 °C and 120 °C and four durations: 0.5, 1.0, 1.5, 2.0 and 4.0 h. Empirical formulae were suggested for correlating the fiber birefringence, the molecular orientation factor, and strain rate at three different stretching speeds; 0.38, 0.57, and 0.77 cm/s. Upon stretching at constant temperature, it is found that the stretching speed has the main effect of controlling the alignment of the polymeric chains in the tested fiber. Birefringence profiles were determined for iPP fibers at different stretching speeds. The average values of maximum (observable) birefringence for iPP fibers were calculated and found to be 0.042, 0.027 and 0.026 for untreated and annealed samples at temperatures of 80 °C and 120 °C, respectively.