Hey Dai, make sure you installed the correct version of CUDA and CUDNN. From tensorflow’s github: “The default support for the tensorflow is cuda 8.0 and cudnn 5" (and cudnn 5.1 should work too).

Also make sure you installed the GPU version of tensorflow.

If you did and tensorflow is still not using the GPU, I suggest you start over by purging CUDA and the nvidia drivers (as described in “Removing CUDA/Nvidia drivers” in the post). Also remove and reinstall tensorflow.