Using the Luo-Rudy I model, the recently proposed T ± ϵ feedback control for suppressing cardiac alternans is investigated. It is shown that cardiac alternans, induced by fast pacing (of period T), can be dramatically reduced by small changes of fixed magnitude (ϵ) under feedback control when ϵ exceeds a critical threshold. Detail information on various ionic currents and cytosolic calcium concentration ϵ([Ca]i) are examined when the cardiac cell is under control. In particular, the alternans in ionic currents and [Ca]i are also reduced under the control. Furthermore, when ϵ exceeds the threshold value, the average [Ca]i level is significantly higher when the alternans is under control, suggesting a possible physiological improvement of cardiac contractions.