Abstract: A constant speed device for a motorcycle having a twist grip throttle and has a clamping segment and a pressing segment. The clamping segment a clamping space, an abutting face and a gap. The clamping space is defined through the clamping segment to mount around the twist grip throttle of the motorcycle. The abutting face is formed around the clamping space of the clamping segment to abut with the twist grip throttle. The gap is formed through a side of the clamping segment and communicates with the clamping space to allow the twist grip throttle mounting in the clamping space of the clamping segment. The pressing segment is connected to the clamping segment opposite to the gap.