The important role of cognitive biases aspects at different stages of information retrieval is mentioned in most information retrieval models. So, it is necessary to understand cognitive aspects of behavior and human interaction. Since one of effective cognitive aspects in information searching is cognitive biases and users experience them, the effect of these biases should be considered. According to this, the purpose of this study is to investigate the role of demographic variables in frequency of cognitive biases in information retrieval. The method of this study is survey; in terms of purpose, this is an applied research implemented in combined method (qualitative and quantitative). The research population was graduate students of the Ferdowsi University (humanities students and engineering students) that were studying at this university in second semester, 1397-1398. In this research, data were collected using the stratified sampling method. The required data for this research was gathered based on log file and semi-structured interview that were based on check list of different cognitive biases.  The content validity of interview questions was evaluated by KIS and psychology specialists and the reliability of both instruments is also correlated with the coefficient of agreement between the opinions of two evaluators that was 0.95. The results showed that in the cognitive biases, including “anchoring”, “attention” and “mere exposure”, there was a significant difference between the frequency of these biases occurrences in users based on gender. Also, in the cognitive biases, including “overconfidence”, “recency”, “attention” and “availability”, there was a significant difference between the frequency of these biases occurrences in users based on grade (M.A. And PhD). Other findings showed that in the cognitive biases, including “regency” and “attention”, there was a significant difference between the frequency of these biases occurrences in users based on field of study (humanities and engineering). Based on the findings of this research, it can be concluded that designers of information retrieval systems and librarians should pay attention to the demographic variables (gender, degree and field of study) in designing and evaluating of information systems because these variables can affect frequency of some biases occurrences in information searching. Of course, the impact of factors like age group, character types, mental schemas, culture and information literacy rate should not be overlooked.