The volume soundness and free linear expansion ratio of hardened cement paste of blended cement with various high calcium fly ashes (HCFA) were tested and the influence of free calcium oxide in HCFA on volume stability of blended cement and its mechanism were studied. The results show that Le Chatelier's expansion value of blended cement increases with the addition of HCFA either raw HCFA or ground HCFA, and the effect of various HCFA on free linear expansion ratio of blended cement is the same as that. When free calcium oxide introduced from HCFA is beyond a limit, the soundness of blended cement may be abruptly undermined. In some designed curing conditions the compensation of dry shrinkage of blended cement paste with HCFA depends on the introduced contents of free calcium oxide from HCFA.