The aim of this paper is to illustrate the impact of teaching supplementary readers on developing EFL learners 'vocabulary. The researcher adopted the descriptive analytical method and used the questionnaire as tool for collecting the study data from the population of the research who were the secondary school English language teachers in Khartoum State, the study sample are 100 English teachers of Khartoum locality secondary schools during the school year (2017-2018). The questionnaire is distributed to them, then recollected and furthermore have been statistically analyzed. The tested hypotheses have shown positive results pertaining the effects of teaching supplementary readers on developing EFL learners' vocabulary. The researcher has concluded the following findings: teaching supplementary readers to secondary school students positively contributes enriching the students’ vocabulary. That is to say teaching supplementary readers has a positive effect in the standard of students. Based on the findings, the study recommends that: Ministry of Education should choose books that are useful, interesting, motivating and suitable for students’ age. Teachers of English language are recommended to encourage their students to study supplementary readers to improve their performance in English language. Teachers of English language should consider the students linguistic abilities and to minimize the difficulties of vocabulary that their student might face during their study to supplementary readers.