<jats:title>Abstract</jats:title> <jats:p>As one of the cooling methods for the blanket in a fusion reactor, forced convection using helium gas has been considered as one of the promising candidates. In this study, in order to clarify the effect of tube length on heat transfer for forced flow of helium gas under various velocities and pressures, experiments on forced convection heat transfer for helium gas in a small diameter tube were conducted. A circular tube made of platinum with an inner diameter of 1.8 mm and a heated length of 50 mm was used in this experiment. The tube was heated with exponentially increasing heat inputs. As a result of the experiment, the heat transfer process can be considered in the quasi-steady state when the e-folding time is larger than about 1.5 s. In addition, the heat transfer coefficient increased with the increases in velocity and pressure. The heat transfer was also higher than that of conventional turbulent heat transfer correlation. By comparing with the experimental results of tube with different heated lengths, it was found that the heat transfer coefficients for the heated length of 50 mm were higher than those of the one with a length of 90 mm.</jats:p>