This study aims to shed light onOman's scientific heritage specifically the linguistic one,take it out to the public and free it from the shelves of stagnation and lost. So that it can see the light again and find insights that receive it, benefit themselves from it and benefit others. Also one objective of this study is to fill the vacancy of Oman's heritage's investigation; as scientific area in Oman lacks the field of investigation. It also shows the role of Omanis in enriching the science of linguistic and grammar because this written documentary in our hands has not previously achieved. It has great importance and it contains useful grammatical, morphological and linguistic information. I have divided this letter into two parts: study part and investigation part. The first part has three divisions; first division includes biography of Al-Jahdhami : his name, his birth, childhood and family. It also includes his scientific journey, his job tasks, his attributes, teachers, students, impacts and death. As for the second part, I specified it for the study. It includes the author's method or approach in the book,his view toward quotation, choices, grammar school, attitude toward some scientists, sources, and defects in the book. I concluded the book with the results of the study. The third division includes description of the written documentary copies and investigation approach. I specified the second part for the book investigation. I attached the technical indexes to the letter, then sources and references. The main results I found from this study as following: 1. His writing style is characterized with ease, convenience and avoiding complexity, he attaches it with everything useful and avoid elaborating to make easy for students. 2. A lot of Quranic evidences and citation as it reaches 73 quotations but no Hadith quotations or Arabs'says and proverbs. 3. Poetic quotations are 61, varied between referred, non- referred and some which are disagreed about its reference. Some of them were quoted completed and parts from others. 4. Most of the says he brought were with adoption, from Sibawayh, Al-Hariri, Ibn Malik sayings and others. He adopted from them without declaration and his views and terms suggest visual trend. 5. Al. Sheikhfall in few defects: abbreviation that negatively affects the sides of the topics he discussed. 6. Sheikh does not have an individual grammatical opinion. 7. He did not fully covered many issues in his study, so I've covered what has missed.