The distribution of Cl in LPCVD film deposited by and was measured by IMMA. Cl was distributed broadly near the interface even in unannealed film. In thick LPCVD films on Si, surface roughness was observed after thermal oxidation. Cl was markedly piled up near the interface between the thermally grown oxide and the Si substrate. In thin films, surface roughness induced by oxidation was not observed. Cl distribution after oxidation was almost the same as that of unannealed film. The Cl pile‐up is not caused by oxidation, but by the existence of the interface. The cause of surface roughness is attributed to a smaller concentration of oxygen than Cl at the interface.