  As a result of observational difficulties, braking indices of only six
rotation-powered pulsars are obtained with certainty, all of which are
remarkably smaller than the value ($n=3$) expected for pure magnetodipole
radiation model. This is still a real fundamental question not being well
answered after nearly forty years of the discovery of pulsar. The main problem
is that we are shamefully not sure about the dominant mechanisms that result in
pulsars' spin-down. Based on the previous works, the braking index is
re-examined, with a conclusion of suggesting a constant gap potential drop for
pulsars with magnetospheric activities. New constrains on model parameters from
observed braking indices are presented.
