Intensity/phase transfer matrices are derived to characterize both cross phase modulation (XPM) and stimulated Raman Scattering (SRS) in IM DD optical fiber systems. Based on the calculation of intensity noise spectrum, comparative study of XPM and SRS induced crosstalk is performed analytically and numerically. Analysis reveals that SRS and XPM leads mainly to low frequency and high frequency noise, respectively; crosstalk from adjacent channels is mainly attributed to XPM while that from channels afar mainly attributed to SRS; and in single span transmission system, crosstalk noise power from XPM will in general exceeds that from SRS.