An AT-cut quartz resonator is disclosed, wherein the length l, the width w and the thickness t of the quartz crystal plate are respectively chosen in the directions of an electrical axis (X-axis), an optical axis (Z'-axis) and a mechanical axis (Y'-axis), the width-to-thickness ratio w/t is chosen in the range of 2.0 to 2.8 and the length-to-thickness ratio l/t is chosen less than 25. Consequently, an AT-cut quartz resonator of miniature size having high Q-values of the major mode and excellent frequency-temperature characteristics of the vibration frequency is realized.