In MPEG, the input pictures can be coded in three different types: I, P, and B. The three pictures require quite different numbers of bits because of different natures of their temporal processing. Hence, an intelligent bit-allocation strategy should assign a picture the number of bits according to the picture's type, as well as the picture organization. This implies a requirement of a prior knowledge of the picture types for a given bit budget. This requirement is not a problem for the encoder, as the encoder can decide the type for each input picture. In fact, the encoder can plan ahead the types of the pictures. A transcoder however has no such a prior knowledge about a picture's type before actually processing the picture. This creates a difficulty in bit allocation for the transcoder. This paper presents a novel rate control scheme for MPEG transcoder that requires no a prior knowledge of the picture types. Our study indicates that the picture target rates determined by the proposed rate control with and without a prior knowledge of the picture types are very close.