The molecular structure of an isotactic polypropylene (PP) was modified by reactive extrusion using peroxide. The reactive extrusion modified was then micro injection molded into a gear and the crystalline behavior of the latter was investigated. The polypropylene used was a homopolymer with a melt flow rate of 2.16 g/10 min (ASTM D1238, 230°C, and 2.16 kg). Dicumyl peroxide (DCP) was used to modify the PP. Its half-life time is about 0.25min at 200°C. The DCP contents were 0.05, 0.1, 0.15 and 0.2wt%, respectively. The extruded threads were pelletized and then dried. The dried pellets were injected into a gear-shaped mold, using a micro injection molding machine (BABYPLAST 6/10P, CRONOPLAST S.L) with a temperature of 225°C and 4MPa injection pressure, as well as an injection speed of 35.1mm/s. The DSC tests were performed on a MDSC2920 differential scanning calorimeter (DSC) in a nitrogen atmosphere. The temperature the test was ranged from 50 to 200°C. The annealing process was performed when the temperature was raised to 200 °C in the DSC and it was kept at that value for five mintues. The samples for DSC tests were taken from the tip of the micro injection molded gear. It was found that the values of the crystallinity of all the injection molded samples were higher than those of the reactive extrusion modified ones. This fact could be attributed to effective molecular adjustment of the PP in the micro-injection molded process. The annealing of the micro injection molded PP led to a further significant increase in crystallility and a decrease in the melting temperature.