This study extensively investigated the surface roughness of injection molded wood plastic composites (WPCs) produced from different amounts of wood flour, polymer matrix, mineral filler, and other additives. A larger range of roughness parameters that used in the previous literature were obtained from nine different WPC compositions by using a robust filtering method (robust Gaussian regression filter) to have a better understanding of the overlaying quality of the WPC samples. Three melt flow index (MFI) of the polymer were tested (MFI 3.6, 12, and 25). It was found that WPCs produced with the polypropylene having a MFI value of 25 were the smoothest. It was noticed that not only the wood flour percentage matters but the combination of wood flour-mineral filler was also important. The WPC compositions with lower polymer amount in favor of more wood flour and mineral filler led to rougher surfaces. A decrease of wood flour in favor of increasing the mineral filler participation percentage had a surface smoothing effect. At the same wood flour content, a decrease in polymer combined with an increase in mineral filler led to rougher WPC surface. Among the tested WPC compositions, the smoothest surface was obtained in the specimens produced from 50% wood flour, 0% mineral filler, and around 40% polymer by weight. The results should be helpful to anticipate the effect on surface roughness of the percentage participation for each amount of the wood or mineral filler, polymer matrix, and additives in further development of WPC combinations.