Abstract: There is a relatively big error in measuring specific heat of waxy crude oil by accuracy calorimeter, the prime cause is two of phase change in process of measurement in calorimeter, i. e. phase change between gas and liquid, and due to wax crystallized out of liquid phase, heat transfer of samples getting worse with the temperature dropping, structure of fin -tube in calorimeter can affect on accuracy of heat metering, the size of gas space in calorimeter on correctness. The paper presents analysis method of optimizing fin -tube number, and discussion of optimal gas space in calorimeter.