  We report the calibration of a gaseous Time Projection Chamber based on
Micromegas charge readout modules with cosmic ray muons, utilizing their
penetrating power and relatively uniform energy deposition per unit length.
Muon events were selected through track reconstruction to characterize detector
performances, such as the drift velocity, electron lifetime, detector gain, and
electric field distortion. The evolution of detector performances with gas
purities and electric drift fields over a 50-day data-taking cycle was measured
by the muon calibration method. For instance, the drift velocity degraded with
gas impurities from ${3.40\pm 0.07 ~ cm/\mu s}$ to ${3.06\pm 0.06 ~ cm/\mu s}$,
and then recovered with gas purification. A ${^{137}Cs}$ calibration source was
also placed inside the detector as a reference for muon calibrations.
