Disclosed are a fixing device and a backlight module using the fixing device. The fixing device includes a positioning post, an embedded part, and at least one elastic part. The embedded part is vertically inserted in the positioning post. The at least one elastic part is disposed at one end of the positioning post. The fixing device of the present invention is capable of enhancing the structure and functions to position accurately. Furthermore, the elastic part is capable of decreasing shocks at the fixed position of the fixing device and improving the problem of stress concentration.