We propose an auto-tuning (AT) method to optimize computational accuracy and power consumption by using a directive-base AT language, named ppOpen-AT. The proposed AT method can optimize arbitrary codes with respect to declared variables and blocks on user programs. The preliminary result by utilizing a simulation code indicates: (1) 1.12 times speedup; (2) 6% energy consumption reduction; are established with maintaining a reasonable computation error by changing a part of computation from double precision to single precision.