A length adjustable worktable includes a main table, a side extension, a guide unit, and an actuating device. The guide unit is engaged with the side extension for guiding the side extension to slide toward or away from the main table. The actuating device includes an operating member, a driving unit, and a positioning member. By manipulating the operating member, the driving unit and the positioning member are driven, such that the side extension is able to slide toward or away from the main table along the guiding unit, thereby allowing the length of the worktable to be adjusted.