The purpose was to survey vocabulary readability of Simorgh (SIM) and Pars Azerakhsh (PAZ) OPACs in two university libraries of Iran. The population included undergraduate and post graduate (except PhD.) students in Ferdowsi (to examine the SIM OPAC) and Shiraz (to examine the PAZ OPAC) universities. Data gathered by a questionnaire from 380 persons of whom 275 (145 of 200 from Ferdowsi and 130 of 180 from Shiraz Universities) returned the questionnaire. Findings showed understanding rate of interface vocabulary was %35.1 for SIM OPAC and %33.9 for PAZ one. Any significant difference was not observed in readability of interface vocabulary between various degree students. Understanding rate of vocabulary used in SIM OPAC interface did not show any significant difference with the same rate in PAZ OPAC. In SIM OPAC and PAZ, a significant difference was observed between vocabulary readability of first page and other pages (for other pages). The impact of users' experience in SIM and PAZ OPACs on their understanding rate of interface vocabulary did not show any significant difference among correct answers obtained from students of different school years. The impact of study areas (humanities, basic sciences and engineering) on vocabulary readability of SIM and PAZ OPACs was examined and results showed a significant difference between means gained from respondents' scores of the three areas in Ferdowsi University (SIM OPAC). In Shiraz University, that impact was not significant. This study was limited to a certain population in two Iranian universities that cannot be easily considered as an representative of the heterogeneous population of academic environments. In addition, the data collected was based on a researcher made questionnaire rather than a standardized one. Most studies in this area are dealing with user satisfaction and emphasize on how much OPAC interfaces satisfy users. However, this paper addresses an aspect of them that has received little attention up - to now. The paper explores users' comprehension of OPAC vocabulary, and the elements that affect it

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