The manufacturing processes of timber screws, which are widely used for wood construction work, include wire manufacturing, heading and threading, case treatment, and coating. A low-carbon steel wire of AISI 1022 is used to easily fabricate timber screws. The majority of case treatment activities are performed to improve the screw strength without affecting the soft, tough interior of the screws in drilling operation. In this study, the Taguchi method is used to obtain optimum case treatment conditions to improve the mechanical properties of AISI 1022 timber screws. The quality of case-treated timber screws is affected by various factors, such as case treatment temperature, case treatment time, atmosphere composition, and tempering temperature. The effects of case treatment parameters affect the quality characteristics, such as case hardness and torsional strength. It is experimentally revealed that ammonia flow rate, case treatment time, and carbon potential are significant for case hardness, while methanol flow rate, case treatment temperature, and case treatment time are significant for torsional strength. The optimum mean case hardness is 673.1 HV, the optimum mean core hardness is 440.0 HV and, for torsional strength, the optimum mean value is 5.3 N·m. The new case treatment parameter settings evidently improve the performance. The strength of the case-treated AISI 1022 timber screws is effectively improved. These results may be used as a reference for fastener manufacturers.