Coverage dependent adsorption of Bi on Si(001) has been studied using scanning tunneling microscope and low-energy electron diffraction. At low coverages, Bi atom adsorbs without breaking the Si(001)-(2X1) structure. At the coverage of approximately 0.5 ML, Bi atoms begin to sit on the troughs between the Si dimer row and form a Bi (1X2) structure with broken Si dimers. Strain due to the large misfit is relieved by the formation of antiphase boundaries and Bi missing line defects.