Background: Phenylalanine is an essential Acid Amine participates in protein synthesis by tyrosine. High levels of phenylalanine in the body lead to the production of large quantities of phenyl ketone, which is excreted through urine and that is why it called as Phenylketonuria. A defect of IQ and attention, visual ability and speech processing in this patient. the aim of this study was hearing threshold evaluation of this patient. Method: In a cross-sectional, descriptive study auditory brain stem responses and otoacoustic emission of patients from 1 to13 years and control group age range of 1 to 14 years evaluated. Results: In 31 patients (62%) delay in ABR waves were outside of the normal range but the overall mean of all waves had in the normal range and the average interval of waves in patients with delay in treatment delay was more than patients group with early treatment. Conclusion: delayed-treatment was faced with more delays in the intervals between ABR waves, although statistically was not significant.