There have hardly been experimental studies of the mechanical properties of Sn-7Zn-0.006Al lead-free solder. The temperature dependence of Young's modulus E and internal friction Q^-1 were investigated in Sn-7Zn-0.006Al, Sn-7Zn, and Sn-8Zn-3Bi solders using flexural vibration. Sn-7Zn-0.006Al solder showed the smallest E drop with increasing temperature, representing heat-softenig. Three Q^-1 peaks appeared at around 90-100°C (1^st), 135-140° (2^nd), and 175-180°C (3^rd) in Sn-7Zn-0.006Al and Sn-7Zn, and two Q^-1 peaks at around 90°C (1^st) and 135°C (2^nd) in Sn-8Zn-3Bi. To investigate the origin of these Q^-1 peaks, vibration experiments at lower frequencies were also carried out. From Arrhenius plots for the Q^-1 peaks in two vibration experiments, the 3^rd Q^-1 peak in Sn-7Zn-0.006Al, the 2^nd Q^-1 peak in Sn-7Zn and those at low frequencies in Sn-8Zn-3Bi were relaxation phenomena obeying the Arrhenius equation, whose activation energies were determined to be 83kJ/mol, 144kJ/mol, and 98kJ/mol, respectively. It was estimated that the origin of these Q^-1 peaks was related to the grain boundary from these values, and that Sn-7Zn-0.006Al alloy was refined in microstructure by the addition of a very small amount of Al and superior to other Sn-Zn system solders with regard to mechanical properties.