A high chair includes a foldable support frame, a chair, and a pair of positioning mechanism. The foldable support frame has a pair of rail struts and a pair of backup struts, the pair of backup struts being pivoted to the pair of rail struts and releasably locked in a use position by a spring-biased coupling sleeve. The chair has a pair of sliding sleeves being capable of sliding alone the pair of rail struts thereby hiding the chair under a dining table. The pair of positioning mechanism are mounted on the chair and associated with a pair of locking elements mounted in the sliding sleeves for releasably locking the sliding sleeves relative to the pair of rail struts, so as to adjust and fix the chair at a selected height.