Many modifications to trapeziectomy have been proposed for the treatment of thumb basal joint arthritis. Their final outcomes have been discussed intensively, whereas functional changes in the early post-operative period have not been fully documented. The purpose of the present study is to evaluate an early functional change following our modified ligament reconstruction with tendon interposition (LRTI) arthroplasty. Nine patients (ten thumbs) were included in this study. Pain levels, strength, and mobility were assessed before and after surgery at intervals of two months. Pain level significantly improved at two months after surgery. Grip and pinch strengths had increased gradually after a temporal decrease at two-month follow-up, and were significantly stronger at six months after surgery. Palmar abduction improved significantly at six months after surgery, whereas opposition did not change significantly. A modified LRTI is an effective procedure in terms of early functional improvement of pain, strength, and mobility.