The aim of the study was to examine the effects of a brief stage-matched smoking cessation intervention group compared with a control group (with usual care) in type 2 diabetes mellitus patients who smoked by randomized controlled trial. There were 557 patients, randomized either into the intervention group (n = 283) who received brief (20- minute) individualized face-to-face counseling by trained nurses and a diabetes mellitus-specific leaflet, or a control group (n = 274) who received standard care. Patient follow-ups were at 1 week, 1 month, 3 months, 6 months, and 12 months via telephone, and assessment of smoking status from 2012 to 2014. Patients smoked an average of 14 cigarettes per day for more than 37 years, and more than 70% were in the precontemplation stage of quitting. The primary outcome showed that both the intervention and control groups had similar 7-day point-prevalence smoking abstinence (9.2% vs. 13.9%; p = 0.08). The secondary outcome showed that HbA1c levels with 7.95% [63 mmol/mol] vs. 8.05% [64 mmol/mol], p = 0.49 at 12 months, respectively. There was no evidence for effectiveness in promoting the brief stage-matched smoking cessation or improving glycemic control in smokers with type 2 diabetes mellitus, particularly those in the pre-contemplation stage.