Objective: To assess the level of YKL-40 in samples of GCF and serum from chronic periodontitis patients with and without diabetes mellitus type II before and after scaling and root planning.Methods: Study was conducted on 92 subjects divided into 4 groups , 23 patients suffering from chronic periodontitis (group 1),23 patients suffering from chronic periodontitis with diabetes mellitus type II(group 2), 23 individuals with clinically healthy periodontium with diabetes mellitus type II(group 3) and 23 individuals with clinically healthy periodontium(group 4), The following clinical periodontal parameters were assessed before treatment to determine the clinical periodontal status in all groups: PD, CAL, GI and PI. And one month after the treatment for all groups, In all groups, GCF and serum samples were collected at the initial visit and after 1month after treatment using filter paper. GCF and serum YKL-40 levels as assessed using ELIZA technique.Results: There was a statistically significant difference between group 2 (diabetic patient with chronic periodontitis) (before treatment) and group 3 (diabetic patient). While there was no a statistically significant difference between group 2 (diabetic patient with chronic periodontitis) (after treatment) and group 3 (diabetic patient), Also we found that there was a statistically significant difference between group 1 (chronic periodontitis) (before treatment) and group 4 (healthy individual) and between group 2 (diabetic patient with chronic periodontitis) (before treatment) and group 4 (healthy individuals) and between group 1 (chronic periodontitis) (after treatment) and group 4 (healthy individual). While there was no a statistically significant difference between group 2 (diabetic patient with chronic periodontitis) (after treatment) and group 4 (healthy individuals), Finally, we found that the reduction of YKL-40 level in patient with chronic periodontitis was 42.67% in GCF and 52.5% in serum. While reduction of YKL-40 level in diabetic patient with chronic periodontitis was 48.3% in GCF and 76.83 in serum. There was no statistically significant difference in percentage reduction of YKL-40 level between the two groups. Conclusion: YKL-40 GCF and serum levels are higher in patients with chronic periodontitis more than healthy individuals, YKL-40 GCF and serum level is higher in diabetic patient with chronic periodontitis more than diabetic patient with healthy periodontium, Elevated GCF and serum YKL-40 levels in diseased groups might suggest that this novel molecule takes part in the inflammatory responses in periodontal disease.