This study is the first in which measurements of thickness, fascicle angle and length, and tendon elongation were combined to examine the impact of eccentric strength training on both muscle architecture and tendinous structures. Eighteen healthy male subjects were divided into an eccentric strength training group (n = 10) and a control group (n = 8). The training program consisted of 18 sessions of eccentric exercises over a 7-week period. All subjects were tested at baseline and after the last training session. Using ultrasound imaging, the fascicle angle and length and thickness of the medial gastrocnemius (MG) were analyzed at rest (i.e., theta(p), Fl(p), and t(p), respectively), at 50% of maximal voluntary contraction (MVC) (i.e., theta(50), Fl(50), and t(50), respectively), and during MVC (i.e., theta(100), Fl(100), and t(100), respectively). Tendon elongation (TE) was measured by tracking the proximal displacement of the myotendinous junction of the MG during ramp isometric contraction. During ramp isometric contraction, the slope of the load-deformation relationship of the gastrocnemius tendon above 50% MVC was defined as an index of stiffness. After training, muscle thickness and fascicle angle increased significantly at rest and during contraction, whereas fascicle length increased at rest and did not change during contraction. Furthermore, the stiffness of the gastrocnemius tendon increased significantly. The results suggest that the behavior of muscle architecture and tendon mechanical properties are affected differently by strength training.