In this work, we investigate analytically quantum mechanical (QM) effects on the threshold voltage (VTH)shift of the surrounding-gate (SO) MOSFETs. We show how VTH is influenced with QM effects with the considerations of (1lO)-silicon (Si) orientation and (100)-Si orientation. When the radius of an SO MOSFET is small «3nm), the VTH shift will be significant, and one should be careful in the use of a device with an extremely small silicon body radius. The analytical results are compared with those obtained by B. Yu et aI., and good agreement is observed.