The present invention provides an image capturing optical system comprising: a positive first lens element having a convex object-side surface in a paraxial region; a second lens element with refractive power; a third lens element; a fourth lens element having a concave image-side surface in a paraxial region; a plastic fifth lens element having a concave image-side surface in a paraxial region, and the image-side surface has at least one convex shape in an off-axis region thereof; and a plastic sixth lens element w having a concave image-side surface in a paraxial region, and the image-side surface has at least one convex shape in an off-axis region thereof.