Atomistic morphology and structure of Si(001) surface exposed to ethylene at room temperature have been studied by scanning tunneling microscopy (STM) and photoelectron diffraction (PED). At an ethylene coverage (θ C ) of ~0.5 monolayer (ML, carbon atom/surface Si-atom), both p(2x2) and c(4x2) structures were clearly resolved in atomistic STM images. The alignments of chemisorbed ethylene of p(2x2) and c(4x2) were confirmed by PED and further shown is that the local structure around chemisorbed ethylene at θ C 0.5 ML is identical to that at the saturation coverage of 1 ML.