A thread ring gauge includes a body having inner and outer peripheries spaced in a radial direction. The body includes a split extending from the outer periphery through the inner periphery, separating the body into two sections respectively having first and second end faces. The first end face includes a groove extending away from the second end face in a width direction perpendicular to the radial direction. The groove extends from the outer periphery towards but spaced from the inner periphery. The groove has a bottom wall facing the second end face. An adjusting member is extended through the first and second sections and movable to adjust a width of the slit between the first and second end faces in the width direction. A gauge block is received in the groove and abuts the bottom wall of the groove and the second end face.