Objectives: The purpose of this study is to examine whether phonological processing skill-based phonics intervention is effective in improving phonological awareness, decoding, and spelling in adults with dyslexia. In addition, this study aimed to ascertain in detail the characteristics of decoding, spelling, and phonological awareness in adults with dyslexia. Methods: The participant of this study was a 46-year-old adult female with dyslexia, and a single subject experiment research method was used. An intervention program for phonological awareness, decoding, and spelling was conducted 28 times in total (3 baseline sessions, 22 intervention sessions, and 3 maintenance sessions) and her performance was assessed with assessments modified by the researcher. Results: It was confirmed that phonological processing skill-based phonics intervention is effective in improving decoding, spelling, and phonological awareness abilities. A detailed analysis of spelling errors in the adult with dyslexia revealed that errors for single vowels, basic consonants, and basic final consonant were the most common. A detailed analysis of errors by phonological awareness unit and task showed struggles with - in order of difficulty - deletion, blending, substitution, and discrimination. Among the discrimination tasks, final consonant discrimination was the most difficult. Conclusion: The results of this study are meaningful as basic data for improving the perception of adults with dyslexia and intervention program for them.