  We show the universal soft behavior of massless particles at the tree level
by considering the operator product expansion of the soft vertex operator with
the hard vertex operators. We find that after eliminating total derivative
terms the world-sheet integral for the soft vertex is determined only by the
regions that are close to the hard verteces. This analyses can be applied to
massless particles in various theories such as bosonic closed string, closed
superstring and heterotic string. When the hard particle is massive we find
that a mixing with different vertex operators occurs.
