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In Taiwan, un-reinforced masonry (URM) was a common type of construction for low-rise buildings due to their low cost and ease of construction. At 1970s and 1980s, an improved type known as “confined masonry” became the mainstream. Confined masonry consists of pre-laid URM brick walls and post-constructed reinforced concrete (RC) boundary beams and columns. It is believed that the RC boundary elements provide confinement to the walls because of shrinkage of concrete. Usually the URM walls in confined masonry are 1-brick thick (24cm), but a confined masonry building is not allowed to exceed 3 stories or 10m high by the Taiwan Building Code. So after 1990s, URM walls are mostly used as pot-laid partition walls for RC buildings. However, there are still many existing confined masonry buildings, include residence, school, and public buildings in Taiwan. Former researches have confirmed the in-plane seismic capacity of URM walls. But in typical school and street side buildings that only have walls in one direction, the out-of-plane direction of the walls becomes the weak direction and the walls fail in their out-of-plane direction before the in-plane strength can fully contributed. Therefore this paper presents an experimental investigation and an analytical approach for out-of-plane behavior and strength of URM walls in-filled in RC buildings subjected to lateral force. The experimental data comes from 2 in-site push over tests of typical school buildings by National Center for Research on Earthquake Engineering.