Abstract As-grown MgB 2 thin films were fabricated by a sputtering method and a co-evaporation method. We found that the critical temperatures strongly depended on the deposition rate. The MgB 2 thin films showed critical temperatures of 29 K for the sputtering method and 35 K for the co-evaporation method with relatively low substrate temperatures. For the fabrication of all MgB 2 SIS junctions, we selected AlN as the material for the barrier layer, which has hexagonal crystal structure. In fabricating MgB 2 /AlN/NbN junctions, we found that the AlN deposition with a higher substrate temperature formed insulator layers other than AlN. We also prepared MgB 2 /AlN/MgB 2 trilayers for SIS junctions using low AlN deposition temperature condition without breaking the vacuum. The MgB 2 /AlN/MgB 2 junctions showed a clear Josephson current and gap structures. The critical current density was 120 A/cm 2 and the ratio of sub-gap resistance and the normal resistance was 3.3 when the AlN insulator thickness was 0.14 nm. We showed current–voltage characteristics of the MgB 2 /AlN/MgB 2 junctions with varying AlN layer thicknesses.