The theory for the time step control has been developed and implemented in the PHISICS/RELAP5-3D© coupled code resulting in three different methodologies and two different ways to define the tolerance. More than 2,000 lines have been added to the PHISICS code and to the RELAP5-3D© interface. The three methodologies have been successfully tested on a simplified model of the Japanese HTTR, obtaining a good speedup while maintaining the relative error for the total power under 1 %. The best methodology for this model seems to be methodology #2 with a flux relative local tolerance of 1e-5. This method allows a speedup factor of 5 with a maximum relative error under 0.3%.