Diacritics restoration is a necessary component in order to develop Arabic text to speech systems. When diacritics are present, the phonetic transcription algorithm can be implemented based on a few rules. Restoring Arabic diacritics based on language model scoring is the dominant approach. A fixed vocabulary is usually used to build the language model used for scoring. Since Arabic is a morphologically rich language, the number of the Out-of-vocabulary (OOV) words is large and the diacritization algorithm fails to restore diacritics for these words. In this letter, we present a novel approach to support open vocabulary diacritics restoration based on the Byte Pair Encoding (BPE) method. The BPE method segments the words into variable length sub-word units and allows open vocabulary from fixed sub-word units dictionary. On the Tashkeela diacritization task, this open vocabulary approach outperforms the word and character based methods commonly used in the literature.