Guided wave based inspection using nonlinear characteristics has attracted great attention for microscale damage in metallic structures. However, the curvature of the pipeline complicates the detection of damage in it. In this study, fatigue testing with a nonlinear signal collecting system and surface-bonded piezoelectric transducers is carried out to identify the fatigue crack in an aluminium pipe using second harmonic as nonlinear wave characteristics. Due to the co-existence of multiple wave modes in a pipe structure, an appropriate nonlinear index is used to describe the nonlinearity. The experimental results show that the breathing behaviour exists only at the early stage of the fatigue crack within 1 mm in length and it can be detected by the method based on nonlinear guided waves. This study provides an effective approach for the measurement of the fatigue crack initiation in pipe-like structures using piezoelectric transducers.