Creep testing of Alloy 718 has been carried out at various loads in the temperature range near 650°C in constant load control mode in order to understand how to predict the creep behavior including tertiary creep. The test results have been used for evaluating the existed models, such as Theta projection and Omega method that have been widely used for predicting long term creep strain and rupture time. After determining variables and material parameters of each method with the test results, estimated creep data from each model have been compared with the each measured creep data from the creep tests. The root cause of the discrepancy between estimated and measured data has been analyzed in order to improve the existed methods. The reliability of the improved model has been evaluated in relation to creep data.