This study aimed to explore Down syndrome EFL learners and their teachers' perceptions of language ability assessment and considering their perceptions in order to develop an appropriate test format which can enable them to present their best of language ability. To achieve this purpose, 35 individuals with Down syndrome (both genders), their teachers, and counselors participated in this research. 21 individuals with Down syndrome were at the basic level of second language development, 14 individuals were elementary. It is noteworthy that four instruments were used in this study: Observation, Interview, and Questionnaire and Down Syndrome EFL learners' language proficiency test. Down syndrome individuals' English classes were also observed to achieve information on the strengths and weaknesses in developing second language. Then, an interview was conducted among Down syndrome individuals, their teachers, and counselors for the purpose of qualitative data required to make a researchers-made questionnaire in order to elicit their assessment perceptions. The data obtained from the study revealed that the Down Syndrome learners preferred to be tested that most of students with Down Syndrome prefer to be assessed only through some especial test items which including the multiple-choice, matching, true-false, short-answer questions, fill-in-the-blanks tests, conversation with patterns and oral assessments. This study can provide teachers and material developers with the knowledge to develop and provide assessment models to help Down Syndrome EFL learners improve their learning quality.