The performance improvement of modern computer systems is usually accompanied by increased computational power and thermal hotspots, which in turn limit the further improvement of system performance. In 3D-ICs, this thermal problem is significantly exacerbated, necessitating the need for active cooling approaches such as micro-fluidic cooling. This paper investigates a co-optimization approach for 3D-IC electric (gate sizing) and cooling design that fully exploits the interdependency between power, temperature and circuit delay to push the powerperformance tradeoff beyond conventional limits. We propose a unified formulation to model this co-optimization problem and use an iterative optimization approach to solve the problem. The experimental results show a fundamental power-performance improvement, with 12% power saving and 16% circuit speedup.