Nowadays, CNC milling process of wood is used commonly. The quality of machined wood is determined by surface roughness. The surface roughness depends on both wood anatomical structure and milling parameters. Different wood modification methods also affect surface roughness of wood. Heat treatment is a wood modification method applied to improve the properties of wooden materials and it has been used commonly in the last decades. The aim of this study was to determine the effect of heat treatment on surface roughness of thermal wood machined by CNC. For this reason, wood samples were obtained from Scots pine (Pinus sylvestris L.), Oriental beech (Fagus orientalis Lipsky.), Uludăg fir (Abies bornmülleriana Mattf.) and Carolina poplar (Populus canadensis) species at 50x50x150 mm dimensions. Heat treatment applications were conducted in a temperature controlled small heating unit. 3 different temperatures (120, 160 and 200 C) and 2 different durations (2 and 6h) were applied to specimens under inert gas (nitrogen) environment. After heat treatment, treated wood has been machined with 8mm of diameter cutter, spindle speed 8000rpm, feed rate 6, 4m/min^-1 and cutting depth 4mm by CNC. Afterwards, surface roughness measurement values have been determined and compared with control samples. According to test results, optimum heat treatment conditions have been stated for minimum surface roughness.