Abstract: Based on the conservation of fluid mass, energy, and momentum, a lumped parameter mathematical model of the helical coiled once-through steam generator for 10 MW High-Temperature Gas-Cooled Reactor (HTR-10) was developed. The steam generator is handled with single tube concept and is divided into three regions as sub-cooled region, boiling region and sup-erheater region. And these regions have movable boundaries to simulate the change of the region length. The steady state and dynamicsimulation results show that the developed model is accurate and effective to simulate the steam generator dynamics.