A reliable test should be consistent and dependable (Brown, 2004). As one of the most important qualities of test usefulness, test reliability is affected by four factors, which are student, scoring, test administration and the test itself (Mousavi, 2002, as cited in Brown, 2004). Since student-related reliability is hard to control, unreliable factors can be easily caused by temporary illness, such as a “bad day”, I will analyze reliability of TOEFL iBT writing only in terms of rater reliability, test administration reliability and test reliability. The TOEFL’s reliability, SEM value is use in the calculation of the reliability estimate for the overall test of 4 skills and is between 3.20 for listening section. The reliability coefficient and is between 0.85 for listening section. Statistical evidence makes the test more reliable.