During the long period service in the SPRR-300 research reactor, the graphite box in the vicinity of the core has been irradiated for about 30 years. In this paper, we take samples from the graphite box and investigate the changes of the thermal, mechanical properties and microstructure, compare them with those of the commercial IG110 and NG-CT-10 graphite. We find that after the long-time service, the low fluence neutron irradiation has induced tremendous irradiation damage defects in the crystal structures of the SPRR-300 graphite. These defects are mainly dislocation loops, stacking faults, voids, microcracks, et al. A degree of amorphization has also been observed. Comparing with those of the IG110 and NG-CT-10 graphite, the coefficients of thermal expansion, thermal diffusivity, compression strength and the flexural strength of the SPRR-300 graphite decrease while the modulus of elasticity in flexure increase. The changes of the thermal and mechanical properties of the SPRR-300 graphite may due to the irradiation-induced defects.