  We investigate the Hopf algebra structure in string worldsheet theory and
give a unified formulation of the quantization of string and the space-time
symmetry. We reformulate the path integral quantization of string as a Drinfeld
twist at the worldsheet level. The coboundary relation shows that the Drinfeld
twist defines a module algebra which is equivalent to operators with normal
ordering. Upon applying the twist, the space-time diffeomorphism is deformed
into a twisted Hopf algebra, while the Poincar\'e symmetry is unchanged. This
suggests a characterization of the symmetry: unbroken symmetries are twist
invariant Hopf subalgebras, while broken symmetries are realized as twisted
ones. We provide arguments that relate this twisted Hopf algebra to symmetries
in path integral quantization.
