In this work, in addition to the sensor application of coated optical microresonator, we show the phase transition of Pd-H system using WGMs. WGMs are propagating on a cylindrical microresonator which is based on a single mode optical fiber coated with a desired metal thickness and desired morphology. Light coupling is achieved by tapered fiber connected to a tunable laser working in the infrared wavelength. WGMs are observed and tracked by transmitted laser light. The sensor consisting of the resonator and a tapered fiber is placed in a metal chamber that is connected to the gas tanks. Desired concentration of the gas is achieved by mixing the carrier gas (nitrogen) and hydrogen. By increasing or decreasing of the hydrogen concentration in the sensing chamber, palladium layer expands or contracts. The change in the radius of the resonator translates in to shifts in spectral positions of the WGMs. However, these expansion or contractions rates are different for different phases of the Pd - H system. For instance, solid solution of hydrogen in palladium, represented by alpha has the lowest expansion or contraction. In contrast beta phase has the highest rate. These phase transitions and intermediate phase are shown using the WGMs.