A power conversion device includes a casing, a middle plate and a converter module. The casing includes a base plate. The base plate has a concave recess therein. The base plate forms a base plate convex hull at a side opposite to the concave recess. The middle plate has a middle plate groove therein. The middle plate groove corresponds to the base plate convex hull. The middle plate and the base plate combine up, such that the base plate convex hull fluidly seals the middle plate groove, making the middle plate groove to form a first coolant channel. The converter module is at least partially located in the concave recess.