Passive buildings conserve energy by decreasing need for HVAC, but it is hard to achieve full thermal comfort unlike air-conditioned buildings. mixed-mode buildings depend on passive mode when thermal comfort is achievable with natural ventilation and switch to active mode otherwise, it can give the building the best of both worlds. Mixed Mode building is a combine of natural ventilated building and AC building which needs different strategy of design which make the Mixed Mode building face a problem in design. this paper aims to reach a strategy to design mixed mode buildings envelope in greater Cairo to reach optimum energy consumption. the research concludes that the MMB is near to be a natural ventilated building with slight heavier south envelope which reach 0.8w/m2.k and 20% WWR although the north façade still a light skin with 2w/m2k and 50% WWR.