A machine for thermally treating and sterilizing pre-packaged food articles using microwaves, which is of a type comprising a treatment tunnel having inlet and outlet ports, a conveyor laid longitudinally of the tunnel interior between the ports to convey food article packages laid into parallel rows, at least one microwave generator having microwave radiators arranged to face the rows of packages, further comprises at least one microwave resonance chamber associated with at least one radiator as an extension thereof and extending at one of the rows, through a chamber wall facing the conveyor there being formed a plurality of slots. The machines enables selective heating of the rows of pre-packaged food articles to make the temperature inside the treatment tunnel uniform.