This study was conducted to analyze grammatical errors in speaking performance made by students grade eleventh in Insan Qurani Islamic Boarding School. It focused on finding out the most commonly grammatical errors committed by students of Insan Qurani and the source of the errors. This study employed descriptive qualitative as the research design. 23 students were participated in this study. To investigate students’ errors, a speaking test was used as the research instrument. The students were required to speak about the given topic, then the researcher made a documentation in form of transcript of their speaking performance to be used as the data of this study. The data were analyzed using Surface Strategy Taxonomy and Richard’s (2002) theory of sources of errors. The result of the study showed that almost all students make four error types, even though in the different percentage. The numbers of the error percentage are as follows: omission 23.3%, addition 14.7%, misformation 60.2%, and misordering 1.70%. These errors were identified to be caused from several factors, those are; interlingual errors 30.7%, over-generalization 15.3%, incomplete application rule 22.7%, ignorance of rule restriction 29.5%, and false concept hypothesized 1.70%. It is indicated that the most commonly grammatical error committed by the students in speaking performance is misformation and the source of the errors is interlingual errors. It was hoped that this research would be useful to improve students’ speaking performance. As findings imply that the teachers of Insan Qurani should give more explanation about grammar specifically in subject-verb agreement and it is suggested for students to apply grammatically good English on regular basis.