Polypropylene (PP) flame retardant composites filled with aluminum hydroxide (Al(OH)(3)), magnesium hydroxide (Mg(OH)(2)) as well as zinc borate (ZB) were prepared with a twin-screw extruder. The melt volume flow rate (MVR) and density of the composites were measured by means of a melt flow rate instrument under experimental conditions with temperature of 180 C and load varying from 2.16 to 5 kg, to identify the effects of the particle size and content. The results showed that MVR of the composites decreased with an increase of the filler weigh fraction (phi(f)) when phi(f) was more than 10 phr. The MVR decreased first and then increased with an increase of the filler diameter (d). The melt density (rho(m)) of the composites increased linearly with an increase of phi(f) and decreased linearly with the increase of d. In addition, the rho(m) increased with an increase of load. Under the same experimental conditions, the MVR decreased slightly while the rho(m) increased somewhat with addition of ZB for the PP/Al(OH)(3)/Mg(OH)(2) composite systems. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 118: 332-337, 2010