We present a 3~5 \mum optical parametric oscillator (OPO) based on ZGP pumped by KTP OPO 2.1-\mum laser. The tuning curves of ZGP OPO are calculated. The 8 \times 6 \times 18 (mm) ZGP crystal, whose end faces are antireflection coated at 2.1 and 3.7~
