Grooved surfaces are widely used to enhance heat transfer. In this study, micro-grooves are applied to miniature channels to improve the evaporative heat transfer of CO2. To evaluate the effect of micro-grooves in miniature channels, heat transfer characteristics are investigated for the smooth multichannel tube with 0.8 mm channel diameter and the grooved multi-channel tube with the same diameter and 8 micro-grooves. The tests were carried out at the evaporation temperatures of 0, 5 and 10C; mass flux between 400 and 800 kg/m2s; and heat flux of 12-18 kW/m2. At lower qualities (less than about 0.3), heat transfer was considerably enhanced by the grooves, however, the micro-grooves showed negative effect in heat transfer at higher qualities. A significant heat transfer enhancement was obtained at high evaporating temperature. Also, heat transfer was enhanced at lower heat and mass flux. The evaporative heat transfer coefficients measured were compared with several correlations. A new correlation was developed to predict the dry-out quality in miniature channel tubes more accurately.