Abstract: Data preprocessing is the basis of the whole process of data mining in Web log mining,which directly influences the quality of the Web log mining and its result. A method of data preprocessing in Web log mining based the user access tree was proposed. The user access tree was created according to the Web logs in the preprocessing and it was used to identify the user and transaction.So the preprocessing can be worked well without the site topology.

Key words: Web log mining, Data preprocessing, User identification, Transaction identification