  We report the discovery of a rare early-type dwarf galaxy (dE), SDSS
J125651.47+163024.2 (hereafter dE1256), possessing a tidal feature that was
likely built up by accretion of an even smaller dwarf galaxy. dE1256 is located
in a nearly isolated environment, at the outskirt of the Virgo cluster. A
detailed morphological examination reveals that the accreted stellar population
is mainly deposited in the outer part of dE1256, where the tidal tail is most
prominent. The inner part of dE1256 is perfectly modeled with a simple S\'ersic
function of index n = 0.63 and half-light radius R$_{h}$ = 0.6 kpc, but in
contrast, the entire galaxy has a size of R$_{h}$ = 1.2 kpc. The mass ratio
between the host and the putative accreted dwarf galaxy is calculated to be
5:1, assuming that the observed two components, inner S\'ersic, and outer tidal
tail residual, represent the host's and accreted galaxy's stellar populations,
respectively. We suggest that while the accretion contributes only 20% of the
overall stellar population, the size of dE1256 grew by a factor of two via the
accretion event. Our results provide, for the first time, strong observational
evidence that a dE is undergoing a two-phase growth, a common phenomenon for
massive galaxies.
