Abstarct: In the present study flow boiling heat transfer in a small size hellicaly coiled tube under constant on-side heat flux condition, experimentally investigated. The helical coil was constructed of a 10-mm ID Stainless stell 310 (SS310) tube with 92-mm coil diameter and 30-mm pitch. Experiments were performed in a range of steam qualities up to 0.1, Reynolds 1900-2700 and heat power 3-5 Kw.Some Single-phase experiments were performed and single-phase heat transfer and pressure drop compared with Well-known corellations. The results showed a good agreement with single-phase correlations. In flow boiling experiments subcooled length, exit steam quality and Two-phase heat transfer under different experiment conditions calculated and results show that by increasing heat power and Reynolds, Subcooled length will be decreased and by increasing heat power and Reynolds Two-phase heat transfer coefficient will be increased which means Boiling heat transfer is dependent on both mass flux and heat flux. In nucleate boiling region by increasing steam quality along tube length, inner side and outer side wall temperature is almost constant. The results also shows that by increasing heat flux and decreasing mass flux steam quality at exit will be increased.