A decision method considering time and power consumption for offloading computations is provided. The method includes: obtaining a computing mission; obtaining a plurality of static effect factors from an effect factor table; collecting a plurality of dynamic effect factors. The method also includes: generating power consumptions and computing time that each of the computing unit and each of the server consumes according to the dynamic effect factors and the static effect factors; calculating cost values of executing the computing mission according to the power consumptions and the computing time; determining a target unit to execute the computing mission according to the cost values, in which the target unit is a computing unit or a server; executing the computing mission by the target unit. Therefore, the power consumptions and the computing time are simultaneously considered, and computations are adequately allocated to one of the computing unit and the servers.