In-tube boiling experiment for R134a is performed in a new type of horizontal three-dimensional (3-D) microfin tube. The effects of mass flux, vapor quality, heat flux and boiling pressure on the heat transfer coefficient are reported. Compared with the data of the smooth tube, the average enhancement ratio is between 2.1 to 2.7. The heat transfer coefficients are higher 30% ∼ 40% than the data in the paper . Based on the experimental data, an empirical correlation is obtained and the maximum deviation is within ±25%.