Abstract: A test data compression scheme based on extended prefix coding is presented.It is a variable-to-variable run length code based on encoding both runs of 0's and 1's.The code word consists of prefix and tail.The scheme uses extended prefix to indicate the type of run.It doesn't add an extra bit.So it can compress test data efficiently.Theoretical analysis and experimental results show that the proposed scheme can provide higher compression efficiency than FDR and has a better adaptability for various test set.Due to using a special shift counter to simplify the control circuit,the decompression circuit can be implemented easily and the hardware cost is low.