This paper describes experimental investigation of high pressure steam-water two-phase flow instability of vertical upward helically-coiled tubes in steam generator. The investigation results show that pressure drop oscillation, density wave oscillation and thermal oscillation are existing in helically-coiled tubes. By means of dimensionless analysis an experimental correlation for predicting stability of system is obtained and stability threshold diagrams are found. At the same time, steam-water two-phase flow instability in vertical downward helically-coiled tubes is also investigated as compared to upward helically-coiled tube in steam generator. At last, two-phase flow instability is predicted for real steam generator.