For wide-ranging applications in nanoscale electronic devices, durable and reproducible p-type nanostructures are essential. In this work, simple ZnO nanowire (NW) p-n homojunctions were grown using a two-step hydrothermal synthesis method. P2O5 served as a doping source to obtain p-type ZnO NWs. The morphology of the ZnO NW arrays was examined using field emission scanning electron microscopy (FESEM). The high resolution transmission electron microscopy (HRTEM) image indicated that the ZnO NW p-n homojunction is single-crystalline with a <0001> growth direction. The distribution of P element was analyzed using energy dispersive spectroscopy (EDS). The dynamic growth observation was conducted using liquid in-situ TEM to investigate the ZnO nucleation and growth mechanism. We divided the ZnO nanocrystal precipitation into three processes. Whether two adjacent particles grow stably or not was found to be related to the distance. Moreover, the temperature-dependent photoluminescence (PL) spectra revealed that two extra emission peaks located at 416 nm and 435 nm were emitted from the ZnO NW p-n homojunction, which resulted from donor-acceptor recombination. In addition, the electron transport properties confirmed the rectification behavior of the multi ZnO NW p-n homojunctions. The turn-on voltage and the current were approximately 2.8 V and 10-4~10-5 A, respectively, under forward bias. The results indicate the potential application of ZnO NW p-n homojunctions as nanoscale light-emitting diodes (LEDs).