Abstract: A well packer or other downhole tool is provided with a combination shear and rotational release mechanism comprising a release member having a set of internal threads selectively engageable with a predetermined number of external threads formed on an elongated tubular mandrel. The release member is keyed to the packer outer housing which in turn is cooperable with the slip and packing element assemblies. The mandrel may be selectively subjected to a predetermined upward pulling force to shear the threads on the release member or, alternatively, the mandrel may be rotated a predetermined number of turns to disengage the cooperating threads on the release member to effect release of the packer from its set position.