A long range transmission network expansion planning (TNEP) for Iran's power system as a developing country is accomplished in this paper. This study is accomplished with energy flow optimizing model (EFOM). One of the challenges for power industry restructuring is to maintain transmission system reliability. To this end, transmission expansion may be inevitable especially for developing countries. However, less attention has been devoted to developing institutional structures and incentives to assure appropriate transmission investments are made in the future, and as a result, transmission planning is becoming a problem of extensive concern. In this paper, some important issues related to transmission planning under demand conditions are extensively discussed.