A clamping apparatus for drilling oblique holes has a clamping device and a guiding device. The clamping device has a positioning set and a clamping set. The positioning set has a positioning arm and an operating handle. The operating handle is pivotally connected to the positioning arm and has a release lever and a pushing lever. The clamping set is pivotally connected to the positioning set and has a clamping arm and an adjusting shaft. The clamping arm is pivotally connected to the positioning arm and has a holding grip. The adjusting shaft is rotatably connected to the holding grip. The guiding device is detachably and adjustably connected to the clamping device and has a guiding mount. The guiding mount is detachably and adjustably connected to the clamping arm and has a guiding hole.