This study aimed to determine the effectiveness of English language as the sole medium of instruction in teaching English subjects by comparing the achievement and confidence of two classes under two different teaching modalities. The study used the two-group Pre-test/Post-test Quasi-experimental Design. The teaching modalities were the use of English Only and Multi-Lingual medium of instruction. Based on the data gathered, the findings showed that the medium of instruction in English class does not affect the achievement of students in English. The students aged 19 and above got a higher score than the students aged 15-18, but the age of the students does not affect their achievement in English. The male students from English Only Class and Multi- Lingual Class got a higher achievement score than the female students; however, the achievement of male and female students is relatively the same. The confidence of the English Only Class and the Multi-Lingual Class are significantly different, however, the age and sex of the students does not affect their confidence in English. The confidence of the Multi Lingual Class students is higher than those students in the English Only Class because the students can express themselves better in their English class than the students in the English Only Class. The findings of the study indicate that the English Only Class achievement is relatively the same than Multi-Lingual Class; though, the confidence of the Multi-Lingual Class students is higher than those students in the English Only Class, thus it implies that the higher the level of confidence of students does not necessarily imply that they would achieve better and the lower the confidence level of students in English class does not necessarily mean that the students will achieve lower; however, this study implied that Filipino students should be exposed to as much English as possible in English classrooms and English teachers should decide in the kind of learning atmosphere that will be established in the classroom, thus increasing the confidence of the students in English language learning.