Si thin epitaxial layers were grown by a temperature modulation Si molecular-layer epitaxy (TM Si MLE) method using Si2H6 gas. The in situ observation of the surface hydrogen desorption reaction during TM Si MLE was systematically investigated under different growth conditions. From experimental results, it was concluded that almost all of the surface hydrogen caused by Si2H6 dissociative adsorption can be released in the case when the modulated temperature is 550 degreesC and retention duration is 0 s. The surface of thin film grown by TM Si MLE was observed by the use of high-resolution field-emission scanning electron microscope (FE-SEM) and perfect selective growth was observed.