Interaction of alkali oxides and SO3 and C3S formation and microstructure was studied using K2CO3 and Na2CO3 as alkali sources and (NH4)2SO4 for SO3. When SO3/K2O=1.43 as mole ratio, K2O and SO3 react to form K2SO4, this phase is immiscible with other oxide melt and thus could not affect C3S formation as well as its microstructure. In a condition of SO3/K2O 1, C3S crystals were round and grown in a much larger size. With addition of Na2O and SO3 by only 1wt% each, C3S formation was strongly hindered. Since C2S was stabilized by Na+ and SO4-2, it could not react to give C3S formation. However in the condition of SO3/Na2O=1.43, a little amount of C3S was formed. It is considered that small amount of Na2SO4 was formed, this phase was immiscible with clinker liquid, and the C3S crystals were formed locally in the liquid part of relatively low Na2O and SO3 compositions. These crystals had irregular and rough surfaces and contained more inclusions than those grown from K2O.SO3 system.