Application-specific instruction set processor (ASIP) has been considered as a good candidate for embedded system design in the past decade. Major research effort has been devoted to customized configuration and instruction set extension for performance improvement with the area constraint. Recent research has taken power consumption and energy efficiency as the second metric, which is critical for embedded and portable devices. However, as the technology scales, power density increases and hence thermal issue has emerged as another critical concern in nanometer circuit designs. In this paper, we factor in the thermal consideration during ASIP synthesis process, and explore the relationship and trade-offs among three dimensions: performance, energy, and temperature. Both the theoretical analysis and the real design implementations of several example custom processors have demonstrated the importance of thermal-aware ASIP synthesis. To the best of our knowledge, this is the first thermal-aware design methodology for ASIPs.