Poly (methyl vinyl ketone) (PMVK) was easily degraded via Norrish Type II reaction by ultraviolet irradiation. Copolymers of MVK with Methyl Methacrylate (MMA) or Styrene (St) have high values in quantum yield of main-chain scission. Therefore, much study has been given to the fine image formation with the copolymers by Deep UV light irradiation. Both copolymers in radical polymerization were limited in the range of 50% for yield to control molecular structure. Photodegradation of the copolymers was measurd by the viscometry. Maximum values of the photodegrading speed were obtained for MVK unit 30 mol % in the copolymer with MMA, and for MVK unit 50 mol % in the copolymer with St, respectively. The photodegradation of the both copolymers have been considered to proceed via main-chain scission by Norrish Type II reaction. The fact was confirmed by measuring IR & UV spectra of the copolymer films which was irradiated with Deep UV light in vacuum and air. Poly (methyl methacrylate) (PMMA) is one of the positive working resists which exhibits the high resolution for Deep UV exposure but its low sensitivity (ca. 500 mJ/cm²) makes the practical use difficult. However, sensitivity of P (MMA-co-MVK)-30, which is MMA copolymer containig 30 mol % MVK unit, is 240 mJ/cm², and then P (St-co-MVK)-37, which is St copolymer containing 37 mol % MVK unit, has high sensitivity of 5-13 mJ/cm². P (St-co-MVK)-37 resist coated on a silicon wafer to 0.4μm thickness resolved 0.5 μm line & space patterns. It was concluded that the photodegradation for St copolymer takes advantage of the well known Norrish Type II reaction, and of the energy migration from St unit to carbonyl group. The high sensitivity is also attributed to St-MVK-St sequence. Moreover, studies of Deep UV resist with MVK copolymers are presented.