Purpose The World Wide Web (WWW) is a major channel of getting information and web search engines are the most popular way of accessing information. This study aims to investigate master students’ mental model completeness level of Google web search engine. Methodology: From the methodological perspective, this research is a practical one based on survey method. The sample population consisted of 30 master students from Ferdowsi University in engineering science and social and humanity sciences, who were selected purposefully and participated voluntarily. The main tools for gathering data were a semi-structured interview (based on Li mental model completeness scale (2007) and designed questions) and direct observation. A modified mental model completeness scale (MMCS) was developed based on Li model, encompassing the perception of (1) the nature of the web search engine, (2) searching features of the web search engine, and (3) the interaction between the searcher and the web search engine. With this scale, a participant’s mental model completeness level was determined by how many components of the first two parts of the scale were described and which level of interaction between the participant and Google was revealed during the searches. Results: The results demonstrated that 80% of the students had incomplete mental models and 20% of them had good mental models and none of them showed a complete mental model. The majority of the students’ perceptionwas good in Specific/General Info, Multiple Type of Databases, Matching Process, Keyword Searching and Info Authority. The majority of the students’ perception was moderate in Restricting/Broadening and Ranking/ Sorting. But the majority of the students’ perception was poor in Limited Info, Indexed Pages/Sites, Search Assistance, Boolean Operators, Search Operators & Prefixes, Punctuation & Symbols, Advanced Search and Search Setting. Also, the results showed that there was a significant difference between mental model completeness level and interactivity level. That is, the students who had the better mental model, showed better interactivity level. This finding supports earlier researches that user behavior or performance is guided by the user’s mental model. Conclusion According to the findings, it is necessary to consider mental model as a factor affecting information seeking behavior, in designing information systems and training users too. Understanding users’ mental models reveal their errors, misconceptions, and knowledge gaps; and by this pathology, it is possible to correct these faults and flaws and increase the effectiveness of systems and trainings.

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