Abstract We investigated the energy rate functions of shallow tremors southeast of the Kii Peninsula in the Nankai subduction zone. Unimodal functions (triangle or bell‐shaped) explain the characteristics of small (<104 J) shallow tremors. As the seismic radiated energy increases, the energy rate functions become more complex and are better characterized by a unimodal function with a sharper peak than a triangle function or with small sub‐events. These tremor characteristics differ from those of ordinary earthquakes. We compared the moment‐duration relationships between shallow and deep tremors and found that shallow tremors have larger seismic moments. This difference can be explained by the thermal conditions along the plate boundary fault.