To study the heat transfer enhancement of flow boiling in "Omega"-shaped grooved tube, an experimental investigation with refrigerant R134a as working fluid was conducted. The experiments were performed at different flow directions with mass flux of 10-30 kg/(m(2).s) and heat flux of 3-10 kW/m(2), respectively. The effects of vapor quality, heat flux and mass flux in horizontal, upward and downward flow were compared and analyzed. The comparative study with smooth tube showed significant enhancement of 1.5-3.3 times on heat transfer coefficient in horizontal and upward flow. Contrary to smooth tube results, heat transfer coefficient of downward flow is much higher than horizontal flow. The comparative study between different flow directions showed that heat transfer coefficient decreased with increasing vapor quality in horizontal and downward flow, however in upward flow, it was almost constant. In horizontal and upward flow, heat flux showed a strong positive effect and the effect of mass flux was not very sensitive; however in downward flow, heat flux showed a negative effect and mass flux showed a stronger positive effect. It is concluded that the effect mechanism of gravity on heat transfer in "Omega"-shaped grooved tube is quite different from smooth tube and heat transfer enhancement caused by "Omega"-shaped grooves is not only because of the increase in inner surface area. What's more, heat transfer is dominated by nucleate boiling in horizontal and upward flow, while in downward flow, convective boiling comes to dominate. (C) 2016 Elsevier Ltd. All rights reserved.