In this paper No.45 steel (equivalent to AISI 1045) was laser alloyed with a special laser alloying powder TH-2. With SEM in situ observation of crack growth from base to alloying zone, the relationship between FCGR and stress intensity factor range (SIFR) was investigated, and the structure morphology of surface crack path and the fatigue fracture was analyzed. It is found that crack would appear on laser alloying zone (LAZ) after a certain cycles, but the boundary of LAZ would retard crack growth. Unstably propagation would occur when the crack from base meet the crack on LAZ. The relationship between FCGR and SIFR was not agreed with Paris formula. Typical fatigue strips was found on fatigue fracture of base and the morphology of LAZ was brittle fracture. It was necessary to design special alloying powder for different base material. To increase ductile phase in LAZ would improve resistance to fatigue crack growth of LAZ.