Aims: The aim of this study was to evaluate the knowledge about GDM and its corresponding relation with glycaemic level in GDM patients. Method: A cross-sectional study was conducted in antenatal clinic of Hospital Pulau Pinang, Malaysia from June 2013 to December 2013 using Gestational Diabetes Mellitus Knowledge Questionnaire (GDMKQ) on the sample of 175 GDM patients. Three most recent fasting plasma glucose (FPG) values (mmol/l) were taken from patients profiles and mean was calculated. Results: A total of 166 patients were included in final analysis. A total mean knowledge score of 166 patients was 10.01 ± 3.63 and total mean FPG value was 5.50 ± 1.13. Knowledge had a significant negative association with FPG (r = − 0.306, P < 0.01). Among different knowledge domains, highest mean score was seen for diet/food values domain and lowest for management of GDM. Educational level seems to be the most significant predictor of GDM knowledge and glycaemic control. Highest mean knowledge score and lowest mean glycaemic levels were recorded for patients aged 25–29 years, Malay ethnicity, working women and family history of DM. Conclusion: Higher Knowledge about GDM is related to better glycaemic control. New educational strategies should be developed to improve the lower health literacy.