The polypropylene–long glass fiber (PP/LGF) composites were prepared using self-designed impregnation device. The mechanical properties and dynamic mechanical properties of PP/LGF composites with two different compatibilizers including PP grafted with glycidyl methacrylate (PP-g-GMA) and maleic anhydride-grafted PP copolymer (PP-g-MA) were investigated on injection molding standard bars. The experimental results demonstrated that the addition of PP-g-GMA led to decrease in mechanical properties and storage modulus. However, it is found that the maximum mechanical properties and storage modulus are reached for the PP/LGF composites when PP-g-MA are 7 wt%. Moreover, PP-g-MA is better than PP-g-GMA in the PP/LGF composites.