The aim of this study was to examine the phonological errors made by dyslexics and non-dyslexic children in Persian orthography which is transcribed with two distinct spelling: words spelled with long vowels (transparent) and words spelled with short vowels (diacritics) are called opaque words. The sample of the study consisted of Fifteen dyslexic students with the mean age of 9.6, (SD= 1.5) and fifteen unimpaired elementary students with the mean age of 9.6 (SD= 1.4) who were Persian residents of 6th district of Tehran. All the participants were controlled for IQ, social class and behavioral disorders. The instrument used in this research was a set of phonological awareness task (rhymed word detection, rhymed word production, first sound detection, final sound detection, blending syllables and syllable deletion). Participants were required to do each of these phonological awareness tasks in order to compare their error rates in transparent and opaque words. The performance of the two groups was examined according to the effect of transparency variable. Data analysis was done using SPSS software. Overall, the finding showed that dyslexic children encountered more problems in both transparent and opaque words than did unimpaired children and the difference was statistically significant (P<0/05). On the other hand, correlation analysis (Pearson) showed a significant negative interaction between two variables (transparency and subjects) (P< 0/001). This means that dyslexic children in compare to normal ones committed more errors in phonological awareness tasks. The results of this research show that the degree of the transparency of writing systems affects the phonological processing ability of individuals.