A thin film semiconductor device has a semiconductor thin film with a film thickness of 200 nm or less. The semiconductor thin film is formed over a dielectric substrate with a warping point of 600.degree. C. or lower. The semiconductor thin film has a region in which a first semiconductor thin film region with the defect density of 1.times.10.sup.17 cm.sup.-3 or less and a second semiconductor thin film region with the defect density of 1.times.10.sup.17 cm.sup.-3 or more are disposed alternately in the form of stripes. The width of the first semiconductor thin film region is larger than the width of the semiconductor thin film region. The grain boundaries, grain size and orientation of crystals over the dielectric substrate are controlled, so that a high quality thin film semiconductor device is obtained.