  We have studied the morphology-density relation and
morphology--cluster-centric-radius relation using a volume limited sample
(0.05$<z<$0.1, $Mr^*<-$20.5) of the Sloan Digital Sky Survey (SDSS) data.
  We found there are two characteristic changes in both the morphology-density
and the morphology-radius relations, suggesting two different mechanisms are
responsible for the relations.
  In the sparsest regions (below 1 Mpc$^{-2}$ or outside of 1 virial radius),
both relations become less noticeable, suggesting the responsible physical
mechanisms for galaxy morphological change require denser environment.
  In the intermediate density regions (density between 1 and 6 Mpc$^{-2}$ or
virial radius between 0.3 and 1), intermediate-type fractions increase toward
denser regions, whereas late-disc fractions decrease.
  In the densest regions (above 6 Mpc$^{-2}$ or inside of 0.3 virial radius),
intermediate-type fractions decrease radically and early-type fractions
increase in turn. This is a contrasting result to that in intermediate regions
and it suggests that yet another mechanism is more responsible for the
morphological change in these regions.
  We also compared the morphology-density relation from the SDSS
(0.01$<z<$0.054) with that of the MORPHS data ($z\sim$0.5). Two relations lie
on top of each other, suggesting that the morphology-density relation was
already established at $z\sim$0.5 as is in the present universe.
