The present study aimed at investigating the effectiveness of a program based on the modified Sheltered Instruction Observation Protocol Model (SIOP) in developing teaching performance of teachers of English. The low level of EFL teachers in teaching performance in the microteaching classes as well as their unawareness of the role that can be played by SIOP Model professional development training in improving their teaching performance was the main reason that urged the researcher to conduct the present study. The subjects of the study were 30 students of General Diploma (the 'one year' system, English section) at the Faculty of Education, Assiut University. The study used the one group pre-post design. Tools of the study included a SIOP Model observation sheet and a SIOP background achievement test. The experiment lasted for 48 hours. t-test was used to analyze the data obtained from the pre-post administration of the tools to determine whether the objectives of the study were achieved. t-test analysis of the data for the pre-and post administrations indicated significantly better post achievement gains. Results revealed that the SIOP program was effective in developing General Diploma students' teaching performance. It was recommended that the SIOP Model should be integrated in the program of preparing teachers of English.