Scan forest is an efficient scan architecture which can reduce the test application cost, test power of scan testing and test data volume greatly. The scan forest is modified for scan-based BIST. Techniques are used to make the existing scan forest architecture to an improved scan forest that is more suitable for BIST. A scan flip-flop regrouping technique is introduced to make the scan flip-flop groups have similar sizes. Sufficient experimental results show that the proposed techniques improve the popular test-per-scan test architecture greatly on fault coverage and test length. It is shown according to the experimental results that test length is reduced 77.3% on average for all benchmark circuits.