This research study investigated the use of writing portfolios and their assessment by raters. In particular it compared the inter-rater reliability of the portfolio assessment criteria currently in use and the new portfolio assessment criteria proposed for Yıldız Technical University, School of Foreign Languages, Basic English Department. The perspectives of the participants on the portfolio assessment scheme and the criteria were also analyzed. This study was conducted at Yıldız Technical University, School of Foreign Languages, Basic English Department in the spring semester of 2005. Data were collected through portfolio grading sessions, focus group discussions and individual interviews. The participants in the study were seven English writing instructors currently working at Yıldız Technical University, School of Foreign Languages, Basic English Department. The instructors scored twelve student portfolios on two different sessions using the criteria customarily used in the institution and the new analytic criteria. Focus group discussions were held before and after the grading sessions. At the end of the grading sessions, instructors were interviewed individually. Grading sessions, focus group discussions and interviews were audiotaped and transcribed. The inter-rater reliability for both of the criteria types was calculated and found to be marginal. The results of the statistical analysis revealed that there was no difference in results of inter-rater reliability between the groups in both of the grading sessions. However, analysis of the focus group discussion and interviews indicated that instructors would appreciate some form of more standardized, analytic and reliable criteria for portfolio grading.