With an input power of 50 W and a gas pressure of 100 Pa, 2.5 (without metal wire) and 10.5 cm (with a copper wire) long microwave air plasma jets are generated, respectively. When 95% Ar + 5% O₂ is instead of air, the length of the plasma jet is stretched to 15 cm. Furthermore, the length of plasma plume is affected differently by adding the processed materials. These different discharge performances originate from the effect of plasma-surface interactions.