Heat transfer performance of a single-phase microchannel heat sink can be enhanced by introducing micro knurls onto its surfaces. To characterize the effects of such a surface modification on the thermal and hydraulic performance, an experimental investigation was conducted on a microchannel heat sink which bottom surface was modified with diamond-pattern micro knurls. The hydraulic diameter of the microchannel was 909 µm. Deionized water was used as the working fluid, and the Reynolds number (Re) for the experiment ranged between 65 and 500. The thermal hydraulic performance was evaluated for different micro-knurling heights (a higher height denoted as R-1 and a lower height denoted as R-2) respectively. Results shows that micro-knurled surfaces are very effective approaches in single-phase microchannel heat transfer. It is seen that both the R-1 and R-2 microchannel heat sinks show better heat transfer performance at any Reynold number than the smoothed microchannel. For the test range, the largest enhancement of thermal performance in terms of average Nu for the R-1 and R-2 cases are around 255% and 68% respectively, comparing with the smoothed microchannel case. The results also show that the height of the micro-knurled surface roughness is an important parameter of the enhancement, as the thermal performance of the R-1 case is much better than that of the R-2 case. The study also shows that the thermal improvement by introducing micro knurls is accompanied with higher pressure drop penalty.