Supercavity, made of supermirrors of almost unity reflectivity, is an important tool in the cavity quantum electrodynamics experiments, where the fundamental interaction of light-matter is studied at the quantum level of single atom and a few photons. The supercavities easily sport cavity quality factors as high as $10^9$ or more. For such studies it is important to characterize a supercavity beforehand, Since its reflectivity is so close to unity, direct, measurement of reflectivity is not possible. Instead, the finesse of tht cavity is measured. In this thesis I have constructed a supercavity with supermirrors coated at 550 nm with an intention to use the finished cavity in the future cavity QED experiments such as an ideal atom detector In my study the cavity length and the cavity decay time were measured and the cavity finesse was obtained from these values. For the decay time measurement, the cavity length was rapidly varied by controlling the mechanical vibrations of the cavity itself. The shape of the cavity transmission signal as a function of time with the probe laser frequency fixed was dependent on the speed of the cavity length change or the cavity scan. For a slow scan the shape was a Lorentzian, but it became an exponentially decaying signal for a rapid scans. For intermediate scan speeds the decaying signal was noticeably modulated. For the control of the mechanical vibration an amplified speaker was used with a function generator. It was found that careful cleaning and preparation of the supermirrors before assembling the cavity was necessary for obtaining ultrahigh finesses. The cleanness of the mirrors was examined under a dark field microscope for various cleaning methods based on the use of different derivatives of alcocol as a cleaning solvent, ways to flush them, and the use of lens tissues. In addition, since the cavity length was a few millimeters, various transverse electromagnetic modes were easily isolated in resonance frequencies, so that the mode patterns of individual modes were easily observed by varying the probe laser frequency.
