Abstract: The steam generators (SG) of high temperature gas-cooled reactor (HTGR) have the characteristics of using helium as coolant in the primary side, once through flow pattern in the secondary side, helical tubes for heat transfer and high working temperature, etc. These make the thermal hydraulic characteristics of HTGR SG differ a lot from the conventional natural circulation SG of pressurized water reactor (PWR). The basic thermal hydraulics and specific thermal hydraulic issues of HTGR SG were described, including the single-phase and two-phase pressure drop and heat transfer calculation in helical tubes, pressure drop and heat transfer of cross flow over helical tube bundles, temperature uniformity and two-phase flow instabilities. The one dimensional steady code, one-dimensional transient code, two-dimensional codes and methods developed by Institute of Nuclear and New Energy Technology (INET), Tsinghua University, for the thermal hydraulic design and for analyzing the temperature uniformity and two-phase flow issues were introduced. The results were presented and discussed. The methods, codes and conclusions can be referenced for other helical tubes or straight tube once through steam generators having similar parameters.