Abstract: As a component of sound transmission through a residence building, flanking transmission has significant effects on the sound insulation wholly. This study investigates the mechanism of the sound flanking transmission through two adjacent rooms within a residence building which are divided into several subsystems by using Statistical Energy Analysis(SEA), and, the sound power equations of these subsystems will be obtained. Then, the sound transmissions on each flanking path are predicted and the dominant sound transmission path is also determined by solving these equations and calculating the total loss factors of the subsystems and coupling loss factors between subsystems. As to a masonry building with heavy-weight homogeneous building envelopes, the results show that(1) non-passing through separating wall flanking transmission paths may become the dominant flanking ones at low frequencies,(2) at medium and high frequencies, all sound transmissions on the flanking paths tend to be consistent, and sound insulation between two adjacent rooms within a residence building only depends on the direct transmission path which passes through the separating wall, and(3) heavy-weight separating wall can dwindle the frequency range of flanking transmission which affects the sound insulation between two adjacent rooms within a residence building. The proposed results can ameliorate sound environment quality and provide the theoretical guidance for sound insulation design in a residence building.