In the Internet world, web search is mandatory to all. Millions of searches performed for each second in a web and that search information are stored in Web Server Log. Web Log data are massive and erroneous stream data. Handling huge data and incorrect data processing are two significant problems in Web Data Mining. Processing of huge data is a complex task, which increases processing time and memory occupation that will reduce the performance of the system. Incorrect data sometimes produce an erroneous result that reduces the quality of data. To handle this problem web Miner's has introduced several preprocessing methods to reduce the log data size and to eliminate incorrect records, which lead to improved quality, quantity and performance.