Abstract: Learning English vocabulary has been considered somehow problematic at all levels. The present research endeavors to investigate the degrees of difficulty in the process of learning of new vocabulary in familiar and unfamiliar contexts for Iranian learners of English as a foreign language. In the first phase, 113 subjects were grouped into three English proficiency levels on the basis of their scores on the TEOFL Test; 40 at pre-intermediate, 38 at intermediate, and 35 at upper-intermediate group. Then they were given two texts to read; one about Halloween, relatively unfamiliar to Iranian students, and another text about Chaharshanbe Suri, a familiar issue for Iranian students. The subjects were to mark all unknown words and, if possible, guess the meanings of words. All successful guessing resulted from contextual clues were crossed out lest it should nullify the whole investigation. Statistical analysis of the participants' performance indicates the following: a) guessing new vocabulary in familiar and unfamiliar texts pose different levels of difficulty; unfamiliar texts being more difficult. b) This difficulty pattern is not affected by the proficiency level of the students.