The melt apparent shear viscosity (eta(a)) of polypropylene (PP) composites filled with aluminum hydroxide [Al(OH)(3)] and magnesium hydroxide [Mg(OH)(2)] was measured by means of a capillary rheometer under experimental conditions of temperature ranging from 180 to 200 degrees C and apparent shear rate varying from 10 to 2 x 10(3) s(-1), to identify the effects of the filler particle content and size on the melt viscosity. The results showed that the melt shear flow of the composites obeyed the power law and presented pseudoplastic behavior. The dependence of eta(a) on temperature was consistent with the Arrhenius equation. The sensitivity of eta(a) for the composite melts to temperature was greater than that of the unfilled PP, and weakened with increasing apparent shear rate. The eta(a) increased linearly with an increase of the weigh fraction of the flame retardant, especially in the low apparent shear rate region. The eta(a) of the composites decreased slightly with an increase of particle size of flame retardant. Moreover, the variation for the eta(a) with particle size of flame retardant was much less than with apparent shear rate under these test conditions. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 1835-1841, 2011