Spelling words accurately is not an easy process. Instead, it is a complex developmental skill. Due to this fact, several studies have recently investigated learners' spelling ability and how this skill develops in different stages. The main purpose of this study is to examine the developmental nature of Saudi intermediate school students' acquisition of spelling knowledge as measured by the "Schlagal's Qualitative Spelling Inventory" (designed to assess spelling error patterns). Further, it aims at determining whether Saudi intermediate school students' spelling performance improves as their grade level increases or decreases based on word-level complexity or difficulty. In fact, this study is significant since it reflects the progress of Saudi intermediate school students not only in spelling, but in other literacy areas such as reading, and listening. A strong parallel has been found between spelling, reading and listening. The sample of the study was 300 participants, all of whom were 7th and 9th male graders who study at intermediate public schools in Riyadh, Saudi Arabia. "Spelling features" and "word spelling" scoring scales were used to score the given data. The findings showed that the 7th-grade students were listed at the "phonetic stage," while the 9th-grade students were listed at the "patterns within words stage" and "syllable juncture stage." Besides, the findings revealed that students' spelling performances improved as their grade level increased. Moreover, students' spelling performances were decreased with the increase of word-level difficulty or complexity.