Background: The Geriatric 8 (G8) is a simplified screening tool to select the appropriate elderly patients for chemotherapy. Vulnerable patients with impaired G8 score might need additional comprehensive geriatric assessment (CGA) with intervention for individual problem. However, the impact of CGA and therapeutic intervention on rate of complete chemotherapy among these patients is rarely addressed. This study aims to evaluate the benefit of CGA guided intervention. Methods: A single center, randomized, open-label study which included newly diagnosed elderly cancer patients (age ≥ 65) with impaired G8 score (≤ 14) who were designated for chemotherapy. After the enrollment, patients were randomized to 1:1 ratio to receive CGA guided intervention (intervention group) or usual care (control group). The primary end point was the rate of complete chemotherapy at 90-day. Associated factors for complete chemotherapy were evaluated. Results: Between June 2019 and December 2019, 52 patients were randomized (26 patients for intervention group and 26 patients for control group). Mean age was 72 years, 59.6% was female, 40.4% had breast cancer and 51.9% had early stage cancer. With G8 assessment, 55.8% had intermediate (score 11-14) and 44.2% had low (score < 11) impaired G8 score. All baseline characteristics were balanced. Using per protocol analysis, there was no significant difference in rate of complete chemotherapy between groups (61.9% vs 50%, OR 1.63; 95%Cl 0.51-5.23; p = 0.42). Considering subgroup analysis in the intermediate G8 score patients, the intervention group had a significant higher rate of complete chemotherapy than control group (81.8% vs 66.7%, OR 2.71; p = 0.02), but no significant difference in low G8 score group (40% vs 27.3%, OR 1.78; p = 0.58). In univariate analysis, age below 75 years, BMI > 20 kg/m2 and intermediate G8 score showed significant factors for improving rate of complete chemotherapy. Conclusions: This is the first study in south-east Asia using CGA and intervention to improve rate of completion in chemotherapy. Although the CGA and intervention had no significant difference but had tendency to be better in completion rate of chemotherapy than usual care. The intermediate-impaired G8 score subgroup is more likely to benefit from CGA guided intervention for complete chemotherapy as planned.