This paper presents the security performance of spectral amplitude code OCDMA systems. We examine factors affecting data security such as system capacity and its influences on the eavesdropper SNR. The eavesdropper probability of error-free code detection for Modified Double Weight (MDW) OCDMA scheme is investigated. The eavesdropper performance is based on observing the authorized transmitted signal based on classical detection theory. For probability of correct detection of 0.5, an eavesdropper receiver would need to detect SNRs of 8, 9.3 and 10 dB for MDW (9, 4), MDW (18, 6) and MDW (30, 8) respectively.