  We use semiclassical method to study closed strings in the modified AdS_5*S^5
background with constant B-fields. The point-like closed strings and the
streched closed strings rotating around the big circle of S^5 are considered.
Quantization of these closed string leads to a time-dependent string spectrum,
which we argue to correspond to the RG-flow of the dual noncommutative Yang
Mills theory.
