A keyboard device for a small size tablet personal computer is provided. The keyboard device includes a first casing, a connecting plate, and a second casing. The first casing includes a first keyboard plate and a first recess. The second casing includes a second keyboard plate and a second recess. The connecting plate is connected with the first casing and the second casing. Consequently, the second casing is rotatable relative to the first casing. The first keyboard plate and the second keyboard plate are collectively defined as a whole keyboard to be operated by both hands of the user. Moreover, the small size tablet personal computer is accommodated within the first recess and the second recess collaboratively.