This paper aims to explore how higher proficiency students delivered peer tutoring programs to help improve lower proficiency students' speaking ability and build up their confidence in English speaking due to limited research studies on peer tutoring, especially in Thailand. Three groups of fourth-year undergraduates, each consisting of a high proficiency user of English as a tutor and three lower proficiency users, participated in the study. Video recordings of each tutoring session, the tutors' record forms, semi-structured interviews and speaking tests were implemented as research instruments. All data were analyzed quantitatively and qualitatively through thematic analysis. The findings showed that a peer tutoring program can enhance the tutees' speaking ability and assist them in gaining more confidence in speaking. Also, the results revealed that the tutees could obtain higher scores in the post-test by applying some of the tutor's techniques. The tutoring sessions helped them learn how to form full sentences and anticipate possible test content, and encouraged them to be willing to speak. This paper suggests that peer tutoring can be promoted either inside or outside the classrooms so as to improve the speaking ability and confidence for low proficiency students.