  In 2011 and 2012, three reactor neutrino experiments, Double Chooz, Daya Bay
and RENO showed positive signals of reactor neutrino disappearance and measured
a mixing parameter sin^2(2th13) at average baselines 1.05, 1.65 and 1.44km,
respectively. It is possible to measure effective Dm31^2 from distortion of
neutrino energy spectrum (E dependence of the oscillation) in those
experiments. However, since it requires a precise energy calibration, such
measurements have not been reported yet. Dm31^2 can also be measured from
baseline (L) dependence of the neutrino oscillation. In this paper, Dm31^2 is
measured from disappearance probabilities of the three reactor experiments
which have different baselines, to be 2.99^{+1.13}_{-1.58}(^{+0.86}_{-0.88}) X
10^{-3}eV^2, where the errors are two (one) dimensional uncertainties. This is
consistent with Dm32^2 measured by \nu_{\mu} disappearance in accelerator
experiments. Importance of Dm31^2 measurement and future possibilities are also
discussed.
