Experiments were performed on the flow boiling of the zeotropic mixture of water-ammonia inside an internally spirally grooved horizontal steel tube with a 12mm average inner diameter. The experimental conditions were the mass fraction of ammonia: 0.95, 1.0 kg/kg, the mass velocity: 40 to 80 kg/(m 2 s), the heat flux: 0 to 20 kW/m 2 and the pressure: 0.7 MPa. The measured heat transfer coefficient reached its maximum as the quality approached about 0.6 but decreased abruptly as the quality increased. This sharp decrease as the quality increased beyond 0.6 may have been caused by mass diffusion resistance that increased the temperature locally at the vapor-liquid interface. The temperature increase at the vapor-liquid interface is discussed by analyzing the phase equilibrium characteristics and is explained in terms of the relationship between the bulk temperature and vapor quality. The heat transfer coefficients are also compared with those for pure ammonia.