Abstract: An effect of Ca on the hardenability,microstructures and impact toughness at room temperature of 45 steel has been investigated.The results show that a trace of Ca in 45 steel behaves towards microalloying.The longitudinal impact toughness at room temperature and hardenability are lowed by an addition of Ca.The austenite grains and interlamellar spacing of pearlite are coarsed,and some intergranular fractures have teen observed in 45Ca steel.AES analysis shows that Ca segregations markedly along austenite grain boundaries.The reasons to affect all of these phenomenons have been discussed by Casegregations on the austenite grain boundaries.