Many horizontal wells are being drilled all over the world to increase productivity in comparison to that of vertical wells. Horizontal wells increase reservoir contact area and thus increase productivity. Therefore it is essential to be able to predict the horizontal well productivity. The major objectives of this investigation are to (1) develop a comprehensive evaluation of currently-used steady-state productivity equations of horizontal wells, (2) investigate the effect of horizontal well length and pay zone thickness on the productivity ratio of horizontal well to vertical well when both have the same drainage area and identical flow conditions, and (3) study the influence of pressure drop on production rate of horizontal wells using different steady-state flow equations. In this study, six different steady-state equations of the open-hole horizontal wells under identical conditions of flow parameters and reservoir geometry are subjected to a wide range of horizontal well lengths (100-3,000 ft), pay zone thickness (50-500 ft), and pressure drop (10-200 psi). These horizontal wells were used to calculate the productivity and study the influences of flow rate and pressure drop on the sensitivity of these equations. The obtained results showed that all steady-state equations provide similar horizontal well productivity for horizontal well length <1,000 ft, while a remarkable difference appears and increases when well length increases >1,000 ft. In addition, the results using all steady-state equations confirmed the recommendation of the application of horizontal wells for thin formations and do not recommend it in thick ones (h >250 ft) for wells having identical rock and fluid properties. Comparing the performance of steady-state equations showed that all of these equations are sensitive to the increase in pressure drop, which yields an increase in the flow rate of horizontal wells. The results of this study have important implications in reservoir simulation studies, since it evaluates some important variables including pay zone thickness, well length, and pressure drop on the performance of each flow equation.