Diabetes is a complex metabolic disorder, which has been increasingly prevalent worldwide. Diabetes is related to increasing the risk of periodontitis determined by glycaemic index control. The glycated hemoglobin (HbA1c) analysis indicates the hemoglobin rate in the blood has been glycated (chemically bonded with glucose) and deter- mines blood glucose levels over the previous three months. The aim of this study was to determine the relationship between glycaemic control in diabetes and the severity of periodontitis. Literature searches were performed using Pubmed with the keyword “HbA1c and periodontitis” and published between 2012 to June 2021. The data from the articles were selected and summarized in tables and a narrative review. A total of 20 articles have been selected based on the relevant topics. Patient with higher HbA1c levels was associated with the severity periodontal diseases determined by several parameters. The parameters were clinical attachment loss (CAL), pocket depth (PD), bleed- ing on probing (BOP), oral hygiene index (OHI), community periodontal index (CPI), periodontal inflamed surface area (PISA), periodontal screening index (PSI), the periodontal pathogen colonized, tooth mobility, and the number of missing teeth. The higher level of HbA1c, indicated poorly controlled diabetes, associated with the increase of periodontitis severity.