Taper- and rodlike Si nanowires (SiNWs) are synthesized successfully on Si and Si0.8Ge0.2 substrates. The Growth mechanisms of taper- and rodlike SiNWs are proposed to be oxide-assisted growth (OAG) and vapor-liquid-solid (VLS) growth, respectively. For taperlike SiNWs annealed at 1200 T for 3 h, the emission peaks are found at 772, 478, and 413 nm. On the other hand, for rodlike SiNWs annealed at 1200 degrees C for 4 h, emission peaks are found at 783, 516, and 413 nin. From the field-emission measurements, the taperlike Si nanowires exhibit superior field-emission behavior with a turn-on field of 6.3-7.3 V/mu m. The field enhancement, beta, has been estimated to be 700 and 1000 at low and high fields, respectively. The excellent field-emission characteristics are attributed to the perfect crystalline structure and the taperlike geometry of the Si nanowires.