Test compression / decompression scheme using variable-length coding, e.g., Huffman coding, is efficient in reducing the test application time and the size of the storage on an LSI tester. In this paper, we propose a model of a decompressor with a buffer for variable-length coding and discuss its property. The embedded buffer allows the decompressor to operate at any input and output speed without a synchronizing feedback mechanism between an ATE and the decompressor. Moreover, we propose a method for reducing the size of the buffer embedded in the decompressor. Since the buffer size depends on the input order of test vectors, test vector reordering can reduce the buffer size. The proposed algorithm is based on fluctuations in buffered data for each test vector. Experimental results show a case where the ordering algorithm can reduce the size of the buffer by 97%.