An upper swing body of an excavator is disclosed wherein a rear protrusion is minimized to reduce an operating radius and a cooling performance is improved even in a narrow engine room. An upper swing body of an excavator according to the present invention comprises a main frame (12) forming a base of the upper swing body; a support (16) including a pair of vertical frames (17, 18) disposed upwardly from an upper surface of a rear portion of the main frame (12) and laterally spaced apart from each other by a certain distance, and a horizontal frame (19) for connecting upper ends of the pair of vertical frames (17, 18) to each other; a cab (80) having a front lower portion supported by the main frame (12), and a rear lower portion supported by the support (16); and an engine (10) disposed in a space formed below the horizontal frame (19) of the support (16).