A chassis structure for an electric power wheelchair adapted for a seat frame to be mounted and supported thereon includes a chassis housing which defines an accommodation space, and an upper cover which covers an upper opened end of the accommodation space. At least one modular raising strip assembly is removably sandwiched between the chassis housing and the upper cover and has a predetermined thickness such that a volume of the accommodation space is increased with the raising strip assembly. With the modular raising strip assembly, the volume of the accommodation space is adjustable according to the battery capacity of a battery pack of a drive unit which is mounted in the accommodation space to meet the user's requirement.