A theoretical study is conducted on an orthotropic solid with a nanodefect (e.g., inclusion, hole, or crack) under far-field antiplane shear loading. A rigorous analytical solution of the stress fields is presented using the Gurtin-Murdoch surface/interface model and a conformal mapping technique. Several new and existing solutions are considered for the special and degenerated cases. The major results of the study are as follows:(1) Interface stresses are greatly dependent on size when the size of a defect is at the nanometer scale, and the interface stresses approach the classical elasticity results when a defect has large characteristic dimensions.(2) The interface effect of a nanodefect decreases with an increase in defect section aspect ratio.(3) When the modulus of the defect (inclusion) increases, the interface effect decreases, i.e., the interface effect can be neglected when the inclusion is sufficiently hard.