Abstract- In this paper, the effect of gas and liquid inlet velocities and for the first time the effect of liquid hold up on slug initiation position are studied experimentally. Empirical correlations are also presented based on the obtained results. The tests are conducted for three liquid hold ups (0.25, 0.50 and 0.75) in a long horizontal channel made of Plexiglas with dimensions of 510 cm2 and 36m length in Multiphase Flow Lab. of Tarbiat Modares University. The superficial liquid and air velocities rated as to 0.11-0.56 m/s and 1.88-13 m/s, respectively. The obtained results show that as αl=0.25, slug initiation position is increasing monotonically with Usl and Usg. During αl=0.50, slug initiation position is increasing with Usl and Usg but the slope is smoother than αl=0.25. For αl=0.75, slug initiation position is decreasing monotonically with Usl and Usg. In the case of equal void fraction of phases, slugs are generated weakly (low pressure). However, for the unequal void fraction of phases strong slugs (high pressure) are formed.