Medicine, IBTO, next to Milad Tower, Tehran, Iran, m.asadi@ibto.ir. Conventional and modern instruction techniques have been introduced in the past decades to improve the learners' performance in academic and professional environments. Content-Based Instruction is powerful, a significant approach, and with a lot of experimental evidences in language education. But, as a crucial point to achieve excellent results, it needs some facilities, qualifications and cooperation. There is a long time

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... approach has been implemented in IRAN. No research has been conducted in related high -schools; while, continuous research to reflect the quality of the recent courses to manage next curriculum is of its' necessities. To evaluate the effect of content based English language instruction in the current situation, this study was conducted. The participants were 81 out of 90 available high school students in primary/ placement test; and in the main study, from 297 available high school boy students 206 had the criteria and participated in the study. KR-21 reliability, paired-samples t-test and Mann-Whitney test in SPSS statistics 22.0 software and quasi-experimental design were used. From the results concluded that there is a positive effect on the students' proficiencies-who were under CBI. But, it is not better than or even equal to the students' proficiencieswho were under conventional instruction. Copy Right, IJAR, 2016,. All rights reserved.