A finite element model of a Jeffcott rotor with cracks was studied. Bas ed on the approach of strain energy release rate, a stiffness matrix of the roto r with a crack was derived. The breathing behavior of the crack was represented considering the opening and closing of the crack using stress intensi ty factor. The unbalance responses of the rotor with transverse and slant cracks were analyzed, as well as the actual breathing behavior of the crack in a steady state rotating cycle. The time of the opening and closing of the slant crack is longer than that of the transverse crack, and the time of the slant crack in f ully closed and fully open states is also longer than that of the transverse cra ck. The surface crack depth increases with 1X and 2X harmonic components of the cracked shaft, but there is a little change of the 3X harm onic component.