As the use of the Internet becomes more popular, a huge amount of information is published on the Web, and users can access the information effectively with Web search services. Since Web search services retrieve relevant documents from those collected by Web robots we need to improve the crawling quality of Web robots. In this paper, we suggest evaluation criteria for Web robots such as efficiency, continuity, freshness, coverage, silence, uniqueness and safety, and present various functions to improve the performance of Web robots. We also investigate the functions implemented in the conventional Web robots of NAVER, Google, AltaVista etc. It is expected that this study could contribute the development of more effective Web robots.