Focusing on the ignorance of co—impacts of network extemality and flexible decision on market-entry，this paper derives the demand curve from consumers’utilities in the market with network externality，and then，under Coumot competition，examines the sole impacts of network externality or flexible decision and their co—impacts on market．entry decision．Based on these，this paper also studies the efects of network externality on the value of a flexible market—entry opportunity．1he results show that under the assumption of geometric Brown motion market and ex post Coumot competition，network externality advances market．entry and raises the value of a flexible market．en． ny opportunity while market-entry flexibility puts off such an entry，rI1I1is implies that an observed entry is the out— eolc／le of the balan cing between the force of network extenudity and that of flexibility to a market—entry decision