The flammability performance and mechanical behaviors for halogen-based and non-halogen-based flame retardant (FR) filled polypropylene (PP) composites were investigated in this study. The halogen-based FR system consisted of a mixture of brominated phosphate ester and antimony trioxide (BR), and the halogen-free FR was a magnesium hydroxide (MH). It was found from limiting oxygen index measurements that 60 wt % of MH was needed in order to achieve the same degree of flammability as the composite containing only 30 wt % of BR. Scanning electron microscopy examinations of the fractured specimens indicated that the interfacial bond strength between PP and MH was stronger than that for PP and BR. The notched Charpy impact strength and the impact fracture toughness were measured and compared. The discrepancies between the two impact test results could be correlated after kinetic energy correction was applied to the Charpy impact strengths.