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In distributed query processing, good estimation algorithms of communication costs are critical for query processing, including distributed XML queries. There are techniques that estimate a communication cost for distributed SQL query processing, and some of techniques are adopted in numerous distributed SQL processors. Therefore adopting the processing techniques for SQL queries for the communication cost-based processing of the distributed XML queries seems natural. Unfortunately, however, the tree-structured XML document is different from the table-shaped relational data. These structural differences make adopting the techniques for SQL queries difficult. This study defines some of the considerations for estimating the communication cost of distributed XML queries, and proposes a method for communication cost-based query processing. The experiments show that the proposed algorithm is reasonable for estimating the communication cost for distributed XML queries.