We report procedure with use of self-assembled silicon carbide (SiC) nanomasks for preparation of ultrahigh-density Si nanotips. Si nanotips with SiC apex may be firstly prepared in a CH4∕H2 plasma treatment and in a subsequent H2 plasma etching a SiC apex may be converted into an amorphous silicon (a-Si) one with an additional function of sharpening the nanotips. A comparative study of the field electron emission from the Si nanotips with apexes of SiC, a-Si and pure Si is carried out, and shows that nanotips with a-Si apexes have not only the highest field enhancement factor but also the best emission uniformity. The physical origins for the above two improvements in field emission are discussed. N.S.X., S.Z.D., and J.C. gratefully acknowledge the financial support of the projects from the NNSF (Grant Nos. 50021202, 90201020, and 60271026), STM (Grant Nos. 2003CB314700, 2002AA313010, and 2001CCA04400), ED of China, the STD, the ED of Guangdong Province, and the STD of Guangzhou City.