A mobility aid includes a carrier unit and a wheelchair unit detachably combined with the carrier unit. The vehicle unit includes a vehicle body having a riding portion, and a first positioning member fixed on the riding portion. An axis extending in the same direction as the length of the vehicle body and passing through the first positioning member is defined. The wheelchair unit includes a body surrounding a body defining an accommodation space, and an axis fixed on the body and located on the axis. The second positioning member moves the second positioning member along the axis toward the first positioning member and cooperates with the assembly when the body moves toward the vehicle body to accommodate the seating portion in the accommodation space. The wheelchair unit is combined with the carrier unit on the first positioning member.一種行動輔具，包含一載具單元，及一可拆離地組合於該載具單元的輪椅單元。該載具單元包括一具有一乘坐部的車體，及一固設於該乘坐部的第一定位件。定義一與該車體長度方向同向延伸且通過該第一定位件的軸線，該輪椅單元包括一圍繞界定出一容置空間的本體，及一固設於該本體且位在該軸線上的第二定位件，在該本體朝該車體移動而使該乘坐部容設於該容置空間的過程中，將驅動該第二定位件沿該軸線朝該第一定位件移動並配合組接於該第一定位件，使該輪椅單元組合於該載具單元。
