PbWO4 and CsI(Tl) crystals are widely used in high energy physics experiments. The photons generated from incident particles in the crystal material are detected by the Avalanche photodiodes (APD) placed at the end of the crystals. In this work, the light generated by 0.1-5 GeV electrons in the crystals has been obtained using with the GEANT4 simulation code. The Single Particle Monte Carlo technique has been used to calculate APD output signals and their fluctuations at a constant avalanche gain value of 50 for both the CsI(Tl) and PbWO4 crystals emission spectrum. The simulation results are agreed well with the experimental results. CsI(Tl) crystal-APD system has provided a good material-device combination. The high signal values and the low signal fluctuations make this combination an excellent choice for scintillating light detection. (C) 2010 Elsevier B.V. All rights reserved.