An experiment of upward gas-liquid two-phase flow was conducted in air-water iso-thermal system under atmospheric pressure. The differential pressure was measured at the fully-developed section by using a variable reluctance type transducer to classify the flow patterns and their transitions. The flow behavior was observed with a high-speed video camera. The probability density function (PDF) of the differential pressure signal was employed to identify the flow pattern. A simplified one-dimensional flow model was proposed to clarify dominant factors affecting the formation and transitions of flow patterns. The model dealt with the gas-component advection based on the spatio-temporal void fraction behaviors by considering the gas compressibility, the wake and the liquid phase redistribution mechanism. The simulation results of the model indicated four kinds of the void wave patterns (ripple-like, rectangular, distorted rectangular and uniform wave patterns) depending on gas and liquid volumetric fluxes. These void wave patterns well corresponded to the experimentally observed flow patterns. The transitions among void wave patterns agree well with Mishima-Ishii flow pattern map. The friction loss estimated by the present model coincides fairly well with Chisholm's empirical formula.