With new technologies, temperature has become a major issue to be considered at system level design. Without taking temperature aspects into consideration, no approach to energy or and performance optimization will be sufficiently accurate and efficient. In this paper we propose an on-line temperature aware dynamic voltage and frequency scaling (DVFS) technique which is able to exploit both static and dynamic slack. The approach implies an offline temperature aware optimization step and online voltage-frequency settings based on temperature sensor readings. Most importantly, the presented approach is aware of the frequency-temperature dependency, by which important additional energy savings are obtained.