Twin-fluid atomizers have been successfully used in many industrial applications. This paper presents experimental studies on internal mixing and atomization in a water-air internal-mixing atomizer. Two-phase mixing process and flow patterns in the internal mixing chamber were visually studied through high speed CCD. Observation reveals that internal mixing was dominated by Gas to Liquid mass Ratio (GLR). As GLR increased, the flow patterns changed from slug flow to annular flow. The Oshinowo and Charles' map can be used to predict the flow patterns for the designed atomizer. Droplet Sauter Mean Diameter (SMD) spatial distributions were measured with Phase Doppler Analyzer (PDA) at different operating conditions. Droplet SMD decreased with the increase of GLR at all operating pressure and locations. In the undeveloped region, a close relationship was observed between flow pattern transformation in internal mixing chamber and droplet SMD distribution, and there was an optimized pressure ranging from 0.2 MPa to 0.3 MPa for atomization since liquid films became thicker under a higher pressure. In the developed region, pressure promoted to generate finer atomization. Possibility Density Function (PDF) distribution of droplet size at different axial locations was analyzed to quantitatively represent the effect of droplet coalescence and breakup. As axial distance increased, PDF of both fine droplets and large droplets decreased. The particle size with the maximum PDF increased with the axial distance as well. The results imply that best atomization performance was acquired in the undeveloped region. (C) 2012 Elsevier Ltd. All rights reserved.