The effect of vapor flow on the falling film evaporation of refrigerant R134a outside a horizontal tube bundle is investigated with an experimental approach. The test space is a cube with a rectangular cross section of 0.575 m (length) x 38.8 mm (width). The tube bundle was 3 x 6 (columns x rows) of staggered horizontal finned tubes made of copper. The longitudinal tube pitch is 22.5 mm and the transverse is 19.9 mm. The external fin density of test tube is 45 fpi (fins per inch), and outside diameter is 19.05 mm. The vapor flow velocity can be adjusted in the range of 0-3.1 m/s. Liquid falling film flow rate ranges from 0.07 to 0.2 kg/m.s. Experiment is firstly conducted at saturation temperature of 6 degrees C without the effect of additional vapor flow at the heat flux of 20, 60, 100 and 180 kW/m(2) (for the first tube row). Vapor flow effect experiment was carried out at three heat fluxes 20, 40 and 60 kW/m(2). It is found that falling film flow rate is an important factor to influence the evaporating heat transfer coefficient. With the effect of vapor flow, both positive and negative effects are observed as the increment of vapor velocity. Positive effects are predominant for the two tubes in the top positions and higher vapor velocity. (C) 2015 Elsevier Ltd. All rights reserved.