  An integrated luminosity of 410 pb^(-1), corresponding to ~ 17 million of eta
events, has been analyzed to measure the eta mass using the decay eta to gamma
gamma. The measurement is insensitive to the calorimeter energy calibration and
the systematic error on the measurement is dominated by the uniformity of the
detector response. As a cross check of the method the pi0 mass from the decay
phi to pi0 gamma, pi0 to gamma gamma has been measured and it is in agreement
with the most accurate previous determinations. The result obtained is m(eta) =
547.873 +/- 0.007 (stat.) +/- 0.031 (syst.) MeV, that is today best measurement
of the eta mass.
