Let R and S be rings with identity, M be a unitary (R, S)-bimodule, and T = ⎛ ⎝ R M 0 S ⎞ ⎠ be the upper triangular matrix ring determined by R, S and M. Let E ij be the standard matrix unit. In this paper we show that every biderivation of T is decomposed into the sum of three biderivations D, and , where D(E 11 , E 11 ) = 0, is an extremal biderivation and is a special kind of biderivation. Using this characterization, we determine the structure of biderivations of the ring T n (R)(n 2) of all n × n upper triangular matrices over R, and show that in the special case when R is a noncommutative prime ring, every biderivation of T n (R) is inner. This extends some results of Benkovič (2009) [1].