A new type of tunable optical fiber based on photonic crystal fibers (PCF), which have a cross section that usually consists of air/silica microstructure that is invariant along the fiber axis, was analyzed. Light can be guided either by modified total internal reflection or by the photonic band gap effect. The capillaries allow for a high interaction between guided light and infused liquid material. Tunable PCFs based on mTIR was demonstrated by pumping a liquid plug across a tapered region. Tunability was explored using the PBG effect by thermally tuning the refractive index of high index liquids.