A modular power device is used for mounting on a main plate. The modular power device includes a first substrate, a driving module and a converting module. The first substrate having a first axial direction and a second axial direction substantially perpendicular to the first axial direction is inserted into the main plate, such that the second axial direction is substantially perpendicular to the main plate. The driving module is located on one side of the first substrate and electrically connected thereon. The converting module is located on the other side of the first substrate and electrically connected to the driving module. A length of the converting module is substantially equal to that of the first substrate in the first axial direction, and a width of the converting module is smaller than a length of the first substrate in the first axial direction.