Background: Diabetes is a major, non-communicable disease with increasing prevalence globally and will be the seventh leading cause of death worldwide by 2030 as estimated by the WHO report in 2011. Saudi Arabia is considered to be one of the highest countries in the Middle East for the incidence of diabetes mellitus. Type 2 diabetes affects the most productive midlife period but has also started to appear in younger age groups. Type 2 diabetes is now increasingly diagnosed among adolescents and young adults, data are lacking regarding knowledge about diabetes mellitus among school students in Saudi Arabia but it is a potentially preventable disease through a combination of lifestyle modification and pharmaceutical treatment. Adopting a healthy lifestyle, including healthy diet choices and exercise habits, plays a critical role in the prevention and control of type 2 diabetes and has been associated with lower risk of type 2 diabetes. Saudi adolescents are at a high risk of developing diabetes as many suffer from obesity, a sedentary lifestyle and hereditary diabetes. In 2004, almost a quarter (23.7%) of the Saudi population was diagnosed with T2DM; this was 10 times the number of diabetic individuals in 1980.6 The occurrence of T2DM has been linked to the high rate of overweight adults (35.5%) in the Saudi population and the number of overweight and obese Saudi adolescents is high among both genders. Aim of the study: To assess the effectiveness of diabetes mellitus knowledge among diabetic in high school students at Makkah Al-Mokarramah in Saudi Arabia 2021. Method: cross sectional study conducted among high school students at Makkah, during the March to May, 2021, the Sample size of diabetic high school students. Our total participants were (500). Results: most of the participants (41.0%) were in the age group more than 16 years, the data ranged from(12.25-17.5) by mean ±SD(15.75±3.112),the majority of them were male (56.%) level of education the majority of participant are secondary level were(65.0%). Conclusion: The level of knowledge of high school students had limited and inadequate knowledge about type 2 diabetes and some of them possessed various misconceptions about this particular chronic disease. Health authorities and school authorities in the region should offer special efforts to improve the level of knowledge of the high students through regular health education campaigns. Diabetes Day should be celebrated in all schools and workshops and lectures given by professionals in collaboration with Ministry of Health on this day.