Monte Carlo simulations have been conducted to investigate the effect of surface roughness on the inverse determination of bulk optical parameters. Results show that mus, mua, and g can be overestimated by an order of magnitude for thin slab tissue samples with a moderate index mismatch at the interfaces if typical surface roughness is neglected. Measurements of Intralipid samples between glass windows with smooth and rough surfaces have been carried out and agreement was found between the numerical and the experimental data. This study suggests that the surface roughness should be taken into account for both in vitro and in vivo determination of bulk tissue optical parameters.