There are not enough psychometrically sound studies about the validity and reliability of the scores obtained from peer assessment. This study examined degree of rater severity drift, a rater effect, in peer assessment. The college students’ presentations were scored by 29 peers in the class using a rating scale. Nine presentations were held on four separate days, two presentations on each of the first three days and three presentations on the fourth day. Drift was investigated with two many-facet Rasch measurement models (separate models and dummy time MFRM). Standardized differences were calculated from the estimates obtained with separate models and interaction terms were calculated with the dummy time MFRM. In drift analysis, shifts in estimations were examined from Day-1 which is a baseline to other three days. Results showed that peer raters varied according to their level of severity and they tend to be lenient. Statistics showed that the quality of the scale was acceptable and its items behaved as expected. In drift analysis, standardized differences and interaction term provided very similar results. Between Day-1 and 2, there was no statistically significant difference in the estimates of the rater severity. Between Day-1 and 3, the percentage of scorers with significant drift in the estimates was 38.10%. The raters’ severity shifts on the average of about 0.14 logit and they displayed more severe scoring behavior. Between Day-1 and 4, the number of raters who had significant shifts in their estimates was three according to the standardized difference method, while one according to interaction method. On the average, the raters became more severe. Among three comparison, Day-4 had the largest rater severity drift on the average; Day 3, however, has the highest number of raters with rater severity drift.