Thesis or dissertation writing is an important and essential task for postgraduate students that must be performed correctly through following the standard guidelines for scientific writing. Any thesis/ dissertation or articles must involve writing a reference list in the last. References must be written correctly and honestly by following one of the well accepted international reference writing style. Many reference styles have been established worldwide and every style may meet the requirement of particular discipline. However, variation in using specific style within specific discipline is usually noticed in many academic institutes. In Iraq, postgraduate students in the field of biology are supposed to use Vancouver style for reference writing in their theses/dissertations. This study hypothesized that error in reference writing may be demonstrated in those theses/dissertations as previous studies showed deterioration in writing process as general. For a period of twelve years 124 fully performed theses/dissertations were analyzed carefully for that purpose. Moreover, interview with 114 postgraduate students in biology field was performed to discuss reference writing process. The results showed that almost all students had neither any idea about types of reference writing styles nor what style they had used in their reference writing. It seems that their style tended to meet the American psychological Association style mixed with Harvard style rather than the Vancouver style. Almost all theses or dissertation revealed different reference writing styles, and even in the same thesis or dissertation or in the same single reference more than one reference writing style was noticed. Many other different errors in reference writing were noticed in every thesis or dissertation. The interview with the students confirmed the results obtained from theses and dissertation analysis, indicating the deficiency in their ability to write reference correctly. This article gives evidences that scientific writing practices and abilities of postgraduate students do not reflect expected quality criteria. Postgraduate students severely lacked the skill and knowledge in scientific writing especially in reference writing. It is highly recommended to introduce extensive courses in their curriculum concerning scientific writing in order to improve the writing process to be scientifically well accepted level by following the standard guidelines for writing.