XML has been established as a format for data exchange on the Internet, which has a large volume scale. By the increase of the volume of XML data, applications need to have a standard query language for efficient retrieving and integrating XML data. XQuery, which is proposed by W3C, is one of the standard query languages for XML data which is widely accepted by many applications. In relation to this, the efficient processing of XQuery queries has become a new research topic and a few researchers have proposed the methods for optimizing XQuery queries to resolve the problem of efficiency. A number of previous researches refer to other information like XML schema or DTD for optimizing the queries. However, our optimization technique does not use any other information except the XQuery query instance itself. Also, some researches just define system dependant optimization methods. In other words, they optimize the execution plan of XQuery queries therefore their methods depend on their XQuery execution engine. However our method can be used in any XQuery engine and in any middleware like a mediator over distributed XQuery engines, because our input is an XQuery query and output also is an optimized XQuery query. The express power of XQuery language is abundant but its syntax is very complex because XQuery borrowed features from several other languages. Therefore, we think that it is necessary to classify the queries and give an optimization method according to the characteristics of classified queries. For these reasons, we consider FOR clauses in XQuery queries because FOR clauses can generate some redundant execution of unnecessary operations. In this paper, we classify the general cases of XQuery queries including FOR clauses, and propose the rewriting rules based on these cases. The focus of our optimization is to reduce the frequency of iterations generated by FOR clauses, of course, without semantic modification of the original query. We implement a prototype system for our optimization algorithm and evaluate the performance between original XQuery queries and optimized XQuery queries.