Due to the diversification and complication of construction projects, controlling risks from the early design-planning phase gives huge impact on success of the construction project. As a part of managing uncertainties it is also important to estimate the project cost several times. Especially, estimating project cost in the early stage gives effects on making a budget for projects. This study estimated the apartment project cost using case-based reasoning(CBR), which is the process of solving new problems based on the past problems. For this, we deduced the apartment cost influence factors which can be gathered in the early stage of project. Based on the factors we established the database for apartment project and calculated the attribute value, attribute similarity and case similarity. Although we retrieve the most similar case from the database, it is very hard to utilize it directly due to the uniqueness of each project. So, Genetic Algorithm(GA) was applied in revising the cost of the retrieved-case. Therefore, the accuracy of the prediction was improved by GA optimization.