Co3O4 mesoporous nanowires with average single crystalline grain sizes of about 8 nm, 12nm, 25 nm, and 45 nm were synthesized by sintering of microwave-assisted hydrothermal processed belt-Co(OH)2 precursors at 300–500 °C for 2 h. Microstructure analysis was conducted by x-ray diffraction, x-ray photoelectron spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM), field emission SEM (FESEM), transmission electron microscopy (TEM), and high resolution TEM (HRTEM) to confirm the composition, structure, and orientation in the nanowires. Systematic magnetic measurements have also been conducted on the nanowires. It was found that the size and orientation have significant effects on the magnetic and exchange bias properties. The interesting finding was made that room temperature ferromagnetism appeared at 350 °C in the high orientation samples. Systematic comparison and analysis of the relationships among the grain size, microstructure, orientation (texture), surface electric structure (O vacancies), and defects with magnetic properties (ferromagnetism, coercive field, exchange bias, etc.) are presented in this work.