A curtain airbag for a vehicle may include: a main chamber installed between a headliner and a vehicle body and provided to be downwardly unfolded to the interior of the vehicle; a diffuser being provided at an inlet portion of the main chamber connected with an inflator, formed with a first outlet and a second outlet which receive and discharge gas from the inflator; and an additional chamber connected with the second outlet of the diffuser and communicated with the second outlet to receive gas from the diffuser and inflated between the headliner and the vehicle body to separate the headliner from the vehicle body. In particular, the diffuser includes a joined portion to close partially or fully the first outlet, and the diffuser increases the gas supply to the main chamber when the joined portion is opened by an internal pressure of the diffuser.