A roll control device for a vehicle curtain has a unidirectional transmission device and a damping assembly. The unidirectional transmission device has a gear base, multiple planet gears and a transmission wheel. The gear base has a connecting segment, a driving segment formed on a side of the connecting segment and an axle. The driving segment has multiple receiving recesses and multiple limiting segments. The planet gears are mounted rotatably and slidably in the receiving recesses respectively and are selectively limited by the limiting segments respectively to rotate with the gear base. The transmission wheel is rotatably combined with the gear base, encloses the planet gears and has a transmission gear. The damping assembly is connected with the unidirectional transmission device and has a driven gear and a damper. The driven gear engages the transmission gear on the transmission wheel. The damper is connected with the driven gear.