The main purpose of this study was to determine the relation between the degree of acquaintance and assessment of other persons’ personality traits and attribution of their behaviour. The second aim was to examine relation between personality traits of observers and their perception of actor’s personality traits and attribution of actor’s behaviour. The study was conducted on a sample of 76 subjects divided into two groups, on the basis of degree of acquaintance with actors. First group estimated a close person, a best friend. Second group estimated a person they are not acquainted with. This person was described in an invented interview. In the first part of the research, all subjects completed The Big Five Inventory (John, 1991). In the second part of the research, subjects estimated personality traits of actors (using BFI) and attributed actor’s behaviour using The Attributional Style Questionnaire (Peterson et al., 1982). The results suggest that there are at least two types of bias in estimation of other people. One type is in relation with acquaintance. Observers estimate close persons more positively than persons they don’t know. Second type of bias is in relation to personality traits of observers. These personality traits are Consciousness, Extraversion and Openness.