In this work the performance of a wavelength division multiple access (WDMA) system with direct detection receiver is investigated. For this purpose, the probability of error in a WDMA network with OOK modulation considering crosstalk, ISI, photo detector noise and thermal noise is calculated and the effect of each on system performance is investigated. The system performance in presence of PIN and APD Photo detectors are compared. Besides the accurate calculations of bit error rate, these calculations have also been performed using Gaussian, Chernoff bound and modified Chernoff bound approximations for the decision variable. Results of our calculations of the photo detector noise penalty in presence PIN and APD photo detectors are presented and compared for different number of channels. Also, from the calculations of the receiver sensitivity, an optimum value for the avalanche gain of APD is obtained. The superiority of APD over PIN photo detection is also demonstrated.