In this paper, we propose a liquid metal tunable antenna for microwave ablation that aims to maintain the optimal impedance matching among different organs of treatment. Tuning was achieved by a pneumatic-driven physical displacement of liquid metal plug inside a dielectric tube interconnecting with the coaxial cable. Simulations were performed at the operating frequency of 2.45 GHz for the tunable coaxial monopole antenna inserted into models of liver, bone, inflated and deflated lung tissue. We demonstrated that with minor tuning of monopole length, the optimal impedance matching can be achieved among a wide range of dielectric property variations.