To investigate the flooding phenomenon in nearly horizontal pipes, experimental studies are performed for air and water countercurrent flow in a test section with a length of 2160 mm, with three different inner diameters of 40, 60 and 70 mm, with different types of end geometry, and with various inclination angles. The effects of the pipe diameter, end geometry and inclination angle on flooding are examined. Two mechanisms governing the transition to flooding are proposed : inner flooding and entrance flooding. The inner flooding is initiated by unstable wave growth, i.e. slugging, at the inner location of the pipe. The entrance flooding is always observed to take place at the entrance of the water flow without slugging. The effect of the inclination angle on flooding is predominant within quite a narrow range of inclination (0 degrees < theta < 1 degrees). A small deviation of the inclination angle from the horizontal plane causes a large deviation of the required air flow rates for flooding. Two local void fractions are measured by parallel wire probes and a scale marked rule in two regions (sub- and super-critical regions). The local effects of the void fraction on slug formation models are investigated. It is found that the differences in the exponents of the void fraction terms of the existing slug formation models mainly result from the definition of the void fraction used. When the void fraction measured in the sub-critical region is used in the slug formation model, Mishima et al.’s model well predicts the trend of the present data. However, if the void fraction measured in the super-critical region is used, Taitel et al.’s model better predicts the present data.