We propose a new method for imaging activation time within three-dimensional (3D) myocardium by means of a heart-excitation model. The activation time is estimated from body surface electrocardiograms by minimizing multiple objective functions of the measured body surface potential maps (BSPMs) and the heart-model-generated BSPMs. Computer stimulation studies have been conducted to evaluate the proposed 3D myocardial activation time imaging approach. Single-site pacing at 24 sites throughout the ventricles, as well as dual-site pacing at 12 pairs of sites in the vicinity of atrio-ventricular ring, was performed. The present stimulation results show that the average correlation coefficient (CC) and relative error (RE) for single-site pacing were 0.9992 ± 0.0008/0.9989 ± 0.0008 and 0.05 ± 0.02/0.07 ± 0.03, respectively, when 5 μV/10 μV Gaussian white noise (GWN) was added to the body surface potentials. The average CC and RE for dual-site pacing were 0.9975 ± 0.0037 and 0.08 ± 0.04, respectively, when 10 μV GWN was added to the body surface potentials. The present stimulation results suggest the feasibility of noninvasive estimation of activation time throughout the ventricles from body surface potential measurement, and suggest that the proposed method may become an important alternative in imaging cardiac activity noninvasively.