The aim of this study is to clarify spatial formation of residential settlements on squatter area by analyzing spatial formation and utilization of houses and open spaces on Borei Keila which is one of the largest squatter areas in Phnom Penh, Cambodia. We conclude the results as below from the data mostly based on the field survey. 1. Simple beds, fire equipment and big water pot are the basic components of interior space. Houses over thirty square meters generally have separate living, dining, private and sanitary rooms. 2. Small open spaces and narrow alleys compose the residential block. Though this residential area was made by each self-built house, definite levels exists on width of the alleys. 3. Around eighty percent of houses have devices such as deep eaves and outside bed to connect interior and exterior space. 4. Outside behaviors are cooking, eating, bathing, washing clothes, discharging, conversation, play, taking rest, sleep, selling and buying, working and others.