The objective of this study was to investigate the effect of filler size on the packing stress and viscosity of uncured resin-composite at 23 degrees C and 37 degrees C. A precision instrument used was designed upon the penetrometer principle. Eight resin-composite materials were tested. Packing-stress ranged from 2.60 to 0.43 MPa and viscosity ranged from 2.88 to 0.02 MPa. s at 23 degrees C. Values for both properties were reduced significantly at 37 degrees C. Statistical analysis, by ANOVA and post hoc methods, were carried out to check any significant differences between materials tested (P < 0.05). Conclusions: Filler size and distribution will affect the viscosity and packing of resin-composites during cavity placement.