Abstract: The present invention provides an orientation adjustment device and an orientation adjustment method of a displayed image. The orientation adjustment method includes the following steps. Step 1 is to capture a first image frame and a second image frame sequentially by an image capturing unit. Step 2 is to obtain a plurality of a first pixel eigenvalues near a first side in the first image frame. Step 3 is to obtain a plurality of a second pixel eigenvalues near the first side in the second image frame. Step 4 is to obtain a difference eigenvalue according to the first pixel eigenvalue and the second pixel eigenvalue. Step 5 is to rotate the image frames output by the image capture unit so that the first side is corresponding to the predetermined display side if the difference eigenvalue is greater than a threshold.