Researchers as well as educators have shown a general interest for investigating techniques that facilitate reading in a foreign language. One such technique that recently has come under closer inspection in the area of second or foreign language acquisition research is glossing. This study aimed to explore the effect of glosses on high school students’ reading comprehension. Subjects of the study were 80 second grade students in a high school in Sarab, East Azerbaijan, Iran. In order to accomplish the purpose of the study, the following tests and materials were used. First, a general English test determined the homogeneity of the subjects. Then a passage serving as the reading comprehension passage with three glossing conditions (textual, visual (pictorial), and textual-pictorial) with the related multiple-choice questions, prepared. In order to become sure that the subjects have no previous familiarity with the words which will be glossed a pre-test was given to the participants. Those students who knew one or some of the words, were excluded from the study. Students were grouped according to their proficiency levels, and were randomly divided into experimental and control groups reading with or without glosses. Data were collected through multiple-choice reading comprehension tests. After the required data was collected, the computer program of SPSS applied. Thus the data was analyzed and interpreted through conducting descriptive statistics. A one-way analysis of variance (One-way ANOVA) was run to determine whether there was significant differences among the performances of 4 groups. A Post-hoc comparison also run to determine the exact differences among the groups. The results indicated that the experimental groups performed significantly better in reading comprehension tests than the control group. This study concluded that different types of gloss facilitated high school student’s reading comprehension. In addition, text difficulty determined student’s use of glosses while reading.