This paper analyzes reactions and evaluations of 70 participants, native and non-native speakers of English to 32 errors written by learners of English as a second language, ESL. It investigates the effect of the academic attainment of ESL teachers on the evaluation of ESL error seriousness. The educational attainment of teachers, both native and non-native, include the B.A., M.A., and Ph.D. degrees. Participants in the study include 34 non-native Palestinian ESL teachers, 26 English native speaking ESL teachers, and 10 English native speakers who are not teachers. Errors in this study are taken from compositions written by Arab-Palestinian students. Eight error categories including prepositions, concord, word order, plural, pronouns, spelling, vocabulary, and verb form are used. Four correct sentences are also included. All participants for the study had to underline errors and evaluate them by indicating the points from 0-5 they would deduct for each error; 5 indicates very serious errors; 0 is for error-free sentences; “1” is for errors which can easily be excused; “2”, “3”, and “4” are means to show intermediate degrees of seriousness. Results of the study show that the three academic groups differ in their evaluation of errors. Whereas the Ph.D. groups are the most lenient, the M.A. groups are the least tolerant. The non-teachers are the most tolerant of all groups.