Cracking in 25Cr35NiNb pyrolysis furnace tubes is difficult to detect in engineering production. This paper presents a metal magnetic memory technique for inspecting cracks. To verify the effectiveness of the proposed technique, positions where abnormal magnetic signals existed were rechecked by penetration testing and scanning electron microscopy. Magnetic memory technique results agree well with penetration testing and scanning electron microscope results, providing an effective method to detect cracks in 25Cr35NiNb pyrolysis furnace tubes. However, further research is required for the magnetic memory technique to attain full maturity for use in engineering applications.