Under different annealing treatment conditions, the low temperature PL properties of unintentionally doped 4H-SiC epilayer have been studied by photoluminescence (PL) technique at 10 K. The results show that there are three emission peaks in the range from 370 nm to 400 nm and the maximum energy is about 3.26 eV, which is in accordance with the energy gaps (Eg) of 4H-SiC at room temperature. The 386 nm and 388 nm peaks (corresponding to ~3.21 eV and ~3.19 eV, respectively) are related with N impurity. When keeping annealing time at 30 min, the PL intensity of 386 nm and 388 nm peaks increases and then decreases with the annealing temperature increasing and reaches a maximum at 1573 K. The PL at 386 nm and 388nm change in quite the same manner with annealing time during isothermal annealing at temperature of 1573 K, whereas the difference is small. With the same annealing treatment, the low temperature PL results of 386 nm and 388 nm coincide with that of intrinsic defects in unintentionally doped 4H-SiC, which results from the interaction of infinitesimal disturbance potential energy between N impurity and native defects.