A curtain airbag is proposed to improve the deployability of an airbag cushion while simplifying the structure of the curtain airbag by forming a sub-chamber. The curtain airbag includes a main chamber of an airbag cushion secured to an upper end inside a vehicle body to be deployed to a lower end thereof, a first sub-chamber of the airbag cushion deployed above a pillar trim while pushing the vehicle body to move the main chamber towards a center of a vehicle, and a second sub-chamber of the airbag cushion deployed between an end of the pillar trim and the main chamber, thus pushing the main chamber to prevent the main chamber from being caught by the end of the pillar trim.