Reliability losses in many electronic systems were identified with the failure of solder joints rather than device malfunctions. The adhesion strength is an important factor for assessing the reliability of the solder joints. In this work, a pull-off test was used to investigate the adhesion strength at the interface of the (100 - x)Sn-x(5Al-Zn) lead-free solders on Cu substrate as-soldered and after thermal cycling, respectively. For the (100 - x)Sn-x(5Al-Zn) solders with the x value increased up to 40 wt%, the adhesion strength decreased from 11.8 +/- 1.5 to 3.3 +/- 0.9 MPa. After thermal cycling (-20-120 degreesC) for 40 cycles, the adhesion strength of 95Sn-5(5Al-Zn) and 91Sn-9(5Al-Zn) solders decreased from 11.2 +/-1.7 to 8.2 +/- 1.3, 7.6 +/- 0.7 to 5.0 +/- 0.8 MPa, respectively. However, the adhesion strength for the solders of 80Sn-20(5Al-Zn), 70Sn-30(5Al-Zn) and 60Sn-40(5Al-Zn) increased from 5.7 +/- 1.7 to 13.3 +/- 1.9 MPa, 4.8 +/- 2.0 to 12.2 +/-1.8 MPa, and 3.3 +/- 1.5 to 16.2 +/- 1.2 MPa, respectively. The formation of intermetallic compound (IMC) is proposed for the enhancement of the strength after thermal cycling in this study.