This letter proposes a run-length code based test data compression technique capable of efficient compression. The proposed test compression method is based on a hybrid run-length encoding, which greatly reduces test data storage on the tester. The code words are carefully selected so as to increase the compression ratio for the test data. Also, a heuristic mapping algorithm and a scan latch reordering method for don't care values in the test cubes increase the compression ratio. Results indicate that the proposed code and heuristic mapping schemes are very efficient in reducing test data. Reduced test data results in less test storage and test time.