In this paper the information transmitting rate of delta-modulation （△M） system is discussed and compared with PCM system. It points out that the principal way of increasing the information rate is to decorrelate the modulating signal. The formulae of computing the signal-to-noise ratio of single integration and double integration delta-modulation systems are given. The signal-to-noise ratios and thier relations to pulse repetition frequency for both systems are calculated with typical modulating signal and the results are compared with PCM system. The views in the problem of compression-expansion with practical meaning for both PCM and △M systems are presented. The △M system is reviewed from the point of view of information theory.