Helically coiled mini-tubes have compact structures and good heat transfer performance which attract wide attentions in applications of compact heat exchangers of miniature thermal systems. In this study, forced convection in the helically coiled mini-tubes is numerically studied for the high-pressure low-temperature argon gas. The distribution of temperature and velocity on the cross-section and the variations of the local Nusselt number along the length and periphery of helical coils are presented. The frictional factors and the average heat transfer coefficients h are obtained for Reynolds number in the range of 22500–82500. In the helical coil with tube diameter of 0.3 mm, the distributed Joule-Thomson effect becomes noticeable, and the secondary flow is more apparent at a large Reynolds number, leading to the enhancement of heat transfer compared with that of 0.5 mm tube. The proposed parametrization of Gnielinski correlation yields good consistency with the numerical results, and the deviations for predictions of friction factor and heat transfer coefficient are within ±10%.