We present a new test data compression and decompression method based on statistic relativity of variable length data block

to resolve the storage and bandwidth problems faced by the automatic test equipment (ATE) in system-on-a-chip (SOC) during test.For each test vector

determine a data block with best relativity to be the referenced data block of it

then compress the vector by the relativity between its data blocks and the referenced data block.Besides

the length of referenced data blocks is independent from each other.Its decompression architecture requires only a finite state machine (FSM)、a five-bit storing device temporarily and a cyclical scan register (CSR) whose length is the same as the referenced data block

so the hardware cost in proposed scheme is small.Experimental results for ISCAS-89 benchmarks show that our new technique can provide a higher compression radios than other compression algorithms.