Thermal management becomes a prominent issue in system design for both server systems and embedded systems. A system could fail if the peak temperature exceeds its thermal constraint. This research studies thermal-constrained scheduling for frame-based real-time tasks on a dynamic voltage/speed scaling system. Our objective is to design speed schedulers for real-time tasks by utilizing dynamic voltage/speed scaling to meet both timing and thermal constraints. Two approaches are proposed: One is based on the minimization of the response time under the thermal constraint, and the other is based on the minimization of the temperature under the timing constraint. We present detailed schedulability analysis for both proposed approaches. Our data show that our proposed proactive approaches outperform existing reactive ones.