Nowadays, the various virtual tests of a vehicle are increasingly on demand to drastically reduce development cycle time. For the purpose of predicting realistic and accurate vehicle response through the virtual test, a tire model precisely representing real tire property is necessary. This study presents an improved UA tire model as a semi-empirical model. This model provides reasonable data fitting even holding the strength of UA tire model. For that purpose, the main formulations for longitudinal and lateral forces are just re-used. However, cornering stiffness, camber stiffness and driving/braking stiffness are experimentally formulated as functions of load to realistically represent tire characteristics. The formulations of friction coefficients are improved, and also friction characteristic parameters are determined as functions of slip angle, and camber angle and load. For the purpose of validation, the improved UA tire model is compared with Magic formula as well as empirical data.