A c(4×4) structure formed by exposing monomethylsilane (MMSi) to a Si(001)-(2×1) surface at substrate temperature of 700^oC was measured using scanning tunneling microscopy (STM). At the stage that the spots originated from c(4×4) structure were brightly observed by reflection high energy electron diffraction (RHEED), both c(4×4) and (2×1) domains coexisted. From the evaluation of the c(4×4) and the (2×1) structures by lineprofile of STM images, the c(4×4) structure was revealed to be contracted, while the distance between the (2×1) dimer rows was expanded. Using X-ray photoelectron spectroscopy (XPS), we have confirmed that the carbon atoms included in MMSi diffused into Si substrate. It can therefore be assumed that the contraction of the c(4×4) surface was originated from the diffusion of carbon into Si subsurface. Because of small lattice constant of SiC compared to Si, the c(4×4) structure was predicted to become the site that enhances the nucleation of SiC islands.