It has been widely observed that the pipe grade high-density polyethylene (HDPE) reveals the discontinuous slow crack growth (SCG) behavior even under the creep loading condition. On the contrary, at the relatively lower level of stress intensity factor (SIF), the crack grows rather continuously. Such characteristics lead to the continuous to discontinuous SCG mode transition behavior when the crack grows from the low SIF level. In this paper, the well-known continuous to discontinuous mode transition behavior during the single SCG process of HDPE is fundamentally solved by modifying the current crack layer (CL) theory. By introducing the suggested energy release rate (ERR)-augmented activation energy reduction concept, the experimentally observed SCG mode transition behavior under the creep loading was successfully simulated. The suggested model is expected to broaden the applicability of the CL model including very low SIF range.