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Large datasets become common in applications like Internet services, genomic sequence analysis and astronomical telescope. The demanding requirements of memory and computation power force data mining algorithms to be parallelized in order to efficiently deal with the large datasets. This paper introduces our experience of grouping internet users by mining a huge volume of Web access log of up to 100 gigabytes. The application is realized using hierarchical clustering algorithms with Map-Reduce, a parallel processing framework over clusters. However, the immediate implementation of the algorithms suffers from efficiency problem for both inadequate memory and higher execution time. This paper present an efficient hierarchical clustering method of mining large datasets with Map-Reduce. The method includes two optimization techniques: Â¿Batch UpdatingÂ¿ to reduce the computational time and communication costs among cluster nodes, and Â¿Co-occurrence based feature selectionÂ¿ to decrease the dimension of feature vectors and eliminate noise features. The empirical study shows the first technique can significantly reduce the IO and distributed communication overhead, reducing the total execution time to nearly 1/15. Experimentally, the second technique efficiently simplifies the features while obtains improved accuracy of hierarchical clustering.