Tendon and ligament are important structures of the human body, since they can sustain tensile loading. The primary function of these tissues is to stabilize and protect the joints they attached to it from distortion and dislocation during daily activities. In general, tendon and ligament have viscoelastic (time dependent) properties that can determine their response to loading and restrict the potential of injuries. The major points that this thesis deal with is the study of the biomechanical behavior of tendon and ligament in response to tensile loading. Moreover, illustrate the mathematical expression that may predict the tendon and ligament behaviour. All of the experiments were carried out on ten rat’s knees for in- vitro tensile test. So that sixty specimens were dissected from the rat knees, twenty specimens for the medial collateral ligaments, twenty for the lateral collateral ligaments and twenty specimens for the patellar tendons which always hydrated to prevent the tissue dryness. Also, the experimental results were applied to the formula of the modified superposition method to predict the biomechanical behaviour of tendon and ligament. The results of the study show the behaviour of the ligament and tendon in response to tensile loading with two techniques; (a) the dead loads technique and (b) the continuous loads technique. The stress- strain relationships were also evaluated and the modified superposition theory was applied at different strain levels to the medial and lateral collateral ligaments and to the patellar tendon. The modified superposition theory gave results that are partly similar to the experimental results. Also, the medial collateral ligament was found to behave similar to the patellar tendon with a difference noticed between the medial and the lateral collateral ligaments. The tendon showed longer initial pattern than that for the ligament due to the presence of higher elastin content in the tendon than in the ligament.