A collapsible stroller has a collapsing operation in width and front/rear directions that can be performed simultaneously. The collapsible stroller has a frame including a pair of front leg tubes and a pair of rear leg tubes pivotally collapsibly coupled to the front legs; a seat unit including a pair of seat side-tubes and a pair of backrest tubes; a first collapsing auxiliary assembly horizontally collapsibly disposed between lateral sides of the frame and operably coupled to the side-tubes of the site unit and the rear leg tubes; a second collapsing auxiliary assembly disposed between the rear leg tubes; and an operation unit operably coupled to the side-tubes of the seat unit and releasibly coupled to the rear leg tubes of the frame, thereby the two dimensional collapsing operation of the stroller can be performed by one hand of the user.