After initial structure of AISI 9260 steel is changed into pearlite and martensite, one is isothermally annealed at $700^{circ}C$ below of $A_1$ transformation point and the other is isothermally annealed at the same condition after 3 cycles of heating and cooling between $680^{circ}C$ and $780^{circ}C$ of $A_1$ transformation point. Analyzing the changes of microstructure, mechanical properties and fractography of tension test, we obtained result as follows. The fastest spheroidization rate by changes of initial structure and heat treatment cycles is appeared at the heat treatment cycle which is isothermally annealed after 3 cycles of heating and cooling at below and above $A_1$ transformation point for martensite. At the above condition, the perfect spheroidization structure is appeared after 60hrs and after then, globular carbide is being coarsened. The mean diameter of globular carbide is $2.4{ imes}10^{-3}mm$ after 90hrs. The changes of tension strength during spheroidization heat treatment follows Orwan function, ${sigma}_o={sigma}_i+Gb/l$, where l is interspacing of carbide particles and at the above condition, ${sigma}_o=70.48+2.5{ imes}10^{-3}/l(kg/mm^2)$. Fractography of fracture of spheroidization structure in tension test is appeared as dimple which is ductile rupture type by nucleation and growth of void, size of dimple is larger and deeper with increasing of heat treatment time.