A foldable stroller includes a rear composite supporting member interposed between and pivoted to two rear supporting rod parts of a frame; the rear composite supporting member can be expanded to hold the stroller in an in-use position, and it can be compressed so as to reduce the width of the stroller when the stroller is not-in-use; the stroller further includes a composite fixing member, which will fix the rear composite supporting member automatically when the rear composite supporting member is moved to the expanded position; the stroller frame has two front supporting rod parts, which each include an upper rod, and a lower rod pivoted to the upper rod; the stroller can be reduced in its height by making the lower rods of the front supporting rod parts become close to the upper rods.