Abstract: The cycloaddition reaction of ketene and formaldehyde, leading to 2-oxetanone, has been studied theoretically by means of RHF/3-21G. This reaction is believed to be nonsynchronous and concerted, taking place through a twisted four membered ring transition state. Two types of frontier orbital interaction are involved in this reaction. The activation barrier is calculated to be 123.1kJ/mol （MP2/3-21G result）.