One of the main requirements for Si-based ultrasmall device is atomic-order control of process technology. Here, we show the concept of atomically controlled processing for group IV semiconductors based on atomic-order surface reaction control in Si-based CVD epitaxial growth. Self-limiting formation of 1-3 atomic layers of group IV or related atoms in the thermal adsorption and reaction of hydride gases on Si(100) and Ge(100) are generalized based on the Langmuir-type model. Moreover, Si or Si 1-xGe x epitaxial growth over N, P or C layer already-formed on Si(100) or Si 1-xGe x(100) surface is achieved and the capability of atomically controlled processing for advanced devices is demonstrated. Additionally, the strain control of the Si 1-xGe x/Si heterostructure due to stripe patterning is discussed.