A seat mechanism of an electric wheelchair characterized by: a front seat plate unit including a front seat plate having front and rear ends that are opposite to each other along a front-rear direction of the electric wheelchair, and left and right guide rails that are elongated in the front-rear direction, that are spaced apart from each other in a left-right direction of the electric wheelchair, that are connected to a bottom surface of said front seat plate, and that extend rearwardly from said rear end of said front seat plate, each of said left and right guide rails having left and right longitudinal surfaces, a top longitudinal surface interconnecting said left and right longitudinal surfaces, a longitudinal rail groove that is indented from at least one of said left and right longitudinal surfaces, and a plurality of positioning holes formed in said top longitudinal surface and longitudinally spaced apart from each other; and a rear seat plate unit including a rear seat plate positioned rearwardly of said rear end of said front seat plate and disposed on said left and right guide rails, two protruding rods each of which protrudes downward from a bottom surface of said rear seat plate to one side of a corresponding one of said left and right guide rails and is elongated in the front-rear direction, a plurality of guide members projecting sideward from each of said protruding rods and longitudinally spaced apart from each other, and a plurality of fasteners each of which extends through said rear seat plate and detachably and selectively engages one of said positioning holes in said left and right guide rails for adjustably positioning said rear seat plate on said left and right guide rails, said guide members of each of said protruding rods being inserted into said rail groove of a corresponding one of said left and right guide rails, such that each of said left and right guide rails straddles and is slidable relative to said guide members of a corresponding one of said protruding rods in the front-rear direction. a front seat plate unit including a front seat plate having front and rear ends that are opposite to each other along a front-rear direction of the electric wheelchair, and left and right guide rails that are elongated in the front-rear direction, that are spaced apart from each other in a left-right direction of the electric wheelchair, that are connected to a bottom surface of said front seat plate, and that extend rearwardly from said rear end of said front seat plate, each of said left and right guide rails having left and right longitudinal surfaces, a top longitudinal surface interconnecting said left and right longitudinal surfaces, a longitudinal rail groove that is indented from at least one of said left and right longitudinal surfaces, and a plurality of positioning holes formed in said top longitudinal surface and longitudinally spaced apart from each other; and a rear seat plate unit including a rear seat plate positioned rearwardly of said rear end of said front seat plate and disposed on said left and right guide rails, two protruding rods each of which protrudes downward from a bottom surface of said rear seat plate to one side of a corresponding one of said left and right guide rails and is elongated in the front-rear direction, a plurality of guide members projecting sideward from each of said protruding rods and longitudinally spaced apart from each other, and a plurality of fasteners each of which extends through said rear seat plate and detachably and selectively engages one of said positioning holes in said left and right guide rails for adjustably positioning said rear seat plate on said left and right guide rails, said guide members of each of said protruding rods being inserted into said rail groove of a corresponding one of said left and right guide rails, such that each of said left and right guide rails straddles and is slidable relative to said guide members of a corresponding one of said protruding rods in the front-rear direction.