A GaN/Si nanoheterojunction is prepared through growing GaN nanocrystallites (nc-GaN) on a silicon nanoporous pillar array (Si-NPA) by a chemical vapor deposition (CVD) technique at a relatively low temperature. The average size of nc-GaN is determined to be～10 nm. The spectral measurements disclose that the photoluminescence (PL) from GaN/Si-NPA is composed of an ultraviolet (UV) band and a broad band spanned from UV to red region, with the feature that the latter band is similar to that of electroluminescence (EL). The electron transition from the energy levels of conduction band and, or, shallow donors to that of deep acceptors of GaN is indicated to be responsible for both the broad-band PL and the EL luminescence. A study of the I–V characteristic shows that at a low forward bias, the current across the heterojunction is contact-limited while at a high forward bias it is bulk-limited, which follows the thermionic emission model and space-charge-limited current (SCLC) model, respectively. The bandgap offset analysis indicates that the carrier transport is dominated by electron injection from n-GaN into the p-Si-NPA, and the EL starts to appear only when holes begin to be injected from Si-NPA into GaN with biases higher than a threshold voltage.