Real estate development requires huge capital investment and a long project period, so, it has very risky characteristics. Typical approaches of real estate development valuation such as NPV(Net Present Value) or IRR(Internal Rate of Return) method have been used so far. But they cannot reflect properly managerial flexibility to adapt and revise decisions in response to unexpected market risks. The real options, which are tailored to deal with uncertainty and flexibility related to an real estate development project, enable managers to take the value of flexibility in investment decision making. Therefore the purpose of this study is to evaluate a real estate development project and suggest optimal investment timing of the project by using binomial option models - especially focusing on office development projects. First, through the literature review and definition of real option, we suggested that a binomial option model which is applicable to real estate development projects. On the basis of the valuation of the project, we indicated an optimal investment timing model using a deferral option. Finally we verified these models through a case study of office development projects.