In this work, a new sun-tracking concept was proposed, and the optical performance of solar panels with such sun-tracking system was theoretically investigated based on the developed mathematical method and monthly horizontal radiation. The mechanism of the proposed sun-tracking technique is that the azimuth angle of solar panels is daily adjusted three times at three fixed positions: eastward, southward and westward in the morning, noon, and afternoon, respectively, by rotating solar panels about the vertical axis (3A sun-tracking, in short). The analysis indicated that the tilt-angle of solar panels, beta(3A), azimuth angle of solar panels in the morning and afternoon from due south, phi(a), and solar hour angle when the azimuth angle adjustment was made in the morning and afternoon, omega(a), were three key parameters affecting the optical performance of such tracked solar panels. Calculation results showed that, for 3A tracked solar panels with a yearly fixed tilt-angle, the maximum annual collectible radiation was above 92% of that on a solar panel with full 2-axis sun-tracking; whereas for those with the tilt-angle being seasonally adjusted, it was above 95%. Results also showed that yearly or seasonally optimal values of beta(3A), phi(a) and omega(a) for maximizing annual solar gain were related to site latitudes, and empirical correlations for a quick estimation of optimal values of these parameters were proposed based on climatic data of 32 sites in China. (C) 2010 Elsevier Ltd. All rights reserved.