Background: Systemic anticancer therapy is indicated in particular patients with early breast cancer (EBC) before or after surgery to eradicate micrometastatic spread and reducing the risk of recurrence. Muscle mass and strength were related to chemotherapy toxicity in cachexia patients. However, there is limited data about the impact of low muscle mass and strength in normal or obese breast cancer patients on chemotherapy toxicity. Methods: A prospective cohort study of women with EBC in whom adjuvant or neo-adjuvant chemotherapy was planned. The subjects were evaluated the muscle mass using bioelectrical impedance analysis, muscle strength using JAMAR hand grips strength, and body mass Index (BMI). The validated cut off of low muscle mass and strength were derived from Indonesian healthy population aged 20-40 years old. The outcome is to know chemotherapy toxicity in the first 3 cycles of chemotherapy, evaluated using National cancer Institute (NCI) common terminology criteria for adverse events version 5.0. The performance status, comorbidity, menopause status, chemotherapy regiment, diet intake, and physical activity were also evaluated to adjust the relationship using logistic regression multivariate analysis. Results: A total of 72 EBC patients were who come to secondary referral cancer center were included. Median of age was 47(25- 59) year old. Most subjects were in normal weight criteria, 26(36.1%) were in obese criteria. The mean of BMI, muscle mass, and muscle strength were 23.73+4.02, 5.61+1.23, 15,32+6.16, respectively. Low muscle mass and strength were observed in 34(55.7%) and 24(39.3%), respectively. Low muscle mass was only associated with chemotherapy toxicity after 3rd cycles of chemotherapy with relative risk (RR) 2.667(1.235-5.757). Low muscle strength was only associated with chemotherapy toxicity after 3rd cycles of chemotherapy with RR 4.206(1.168-15.143). After adjusted with performance status, comorbidity, menopause status, chemotherapy regiment, diet intake, and physical activity, low muscle mass and strength were associated with chemotherapy toxicity after 3rd cycles of chemotherapy with odds ratio 11(1.730-69.956) and 14.625(2.191-97.612), respectively. Conclusions: Low muscle mass and strength were associated with chemotherapy toxicity after three cycles of chemotherapy in women with EBC.