Ultra-violet (UV) lasers have become more important tools for various industrial applications, such as laser cleaning and micro-machining, i.e., plastics marking and hole drilling in circuit boards. Diode-pumped frequency-quadrupled Nd:YAG laser is a suitable source with high repetition rate, high efficiency, compactness, and low utility consumption.The major challenge to such an UV laser seems to depend on the ability and reliability of nonlinear optical crystals. Although f3-BaB'O, (BBO) has a large effective nonlinear optical coefficient, disadvantages such as large walk-off angle, narrow angular and temperature allowances, and low damage threshold limit the available UV power. So far we have reported CsLiB,O,, as a promising candidate of UV generator'^',^. In this work we have investigated the theoretical and experimental conversion efficiency and reliability of CLBO compared with BBO, and examined the applicability of micro-machinig.The fundamental source was a diode-pumped high repetitively Nd:YAG laser (SpectraPhysics T40-X30-106Q). Q-switched pulses at 1.064 p m were converted to 0.532 y m (SHG) in a noncritically phase-matched type-I LiB,05 (LBO) at 148 "C, then 0.532 p m to 0.266 y m (FHG) in a type-I CLBO or a type-I BBO. CLBO crystals were kept continuously at 150 "C to relieve stresses introduced by crystal hydration, cutting, polishing, and thermal shock owing to laser power absorption'. The crystal lengths were 12, 10, and 8 mm, respectively. The LBO and BBO crystals with dual-AR coatings were obtained from CASIX, Inc. CLBO crystals have no coating.. This low-peak-power pulsed laser required tight focusing to achieve effective harmonic generation. For theoretical calculation of conversion efficiencies, we employed the focused beam analysis4 of Boyd and Kleinman and its extended theory5. We