Common methods of determining the spatial resolution of fiber-optic image guides are by measuring the diameter of individual microfibers or by the use of a resolution test target. However these methods cannot provide enough information of spatial resolution in ultrathin fiber-optic image guides. In this study, a simple method to measure the modulation transfer function (MTF) of an mage guide was developed. The MTFs of ultrathin image guides with 3 and 4${mu}{ extrm}{m}$ Um diameter were measured by examining transmitted sharp edge image. This method should be especially useful for measuring spatial resolution of ultrahigh resolution image guides with less than 5 ${mu}{ extrm}{m}$ diameter microfibers because their spatial resolution cannot be determined by individual microfiber diameter due to crosstalk and leaky ray phenomena.