Abstract: There are three methods as wetting phenomena in penetrant testing: adhesion wetting, immersion wetting, spreading. The adhesion wetting will occur on the condition that the contact angle θ on the surface of the specimen is less than or equal to 180°. If θ is less than or equal to 90°, immersion wetting will happen. Spreading occurs when θ≈0°. The requirement of spreading is the highest; immersion wetting is in the middle; the adhesion wetting comes to the last. Penetrant testing needs spreading. The contact angle θ is determined by the interfacial tension of gas, liquid and solid. The degree of wetting can be known by the value of θ. These discussions will help to understand the practical problem of the Penetrant testing.