Although health effects of air pollutants are well documented in many countries especially in North America and Western Europe, few studies have been conducted in Thailand where pollution mix, weather conditions, and demographic characteristics are different. The present study aimed to investigate the effects of ambient air pollution on hospital admissions for cardiovascular and respiratory diseases in Bangkok, Thailand. We obtained daily air pollution concentration (O3, NO2, SO2, PM10, and CO) and weather variable monitored in Bangkok from January 2006 to December 2014. Daily hospital admissions for cardiovascular and respiratory diseases were obtained from the National Health Security Office during the study period. A time-series analysis with generalized linear model was used to examine the effects of air pollution on hospital admissions by controlling for long-term trend and other potential confounders. The effect modification by age (0–14 years, 15–64 years, ≥65 years) and gender was also examined. An increase of 10 μg/m3 in O3, NO2, SO2, PM10, and 1 mg/m3 in CO at lag 0–1 day was associated with a 0.14% (95% CI: −0.34 to 0.63), 1.28% (0.87 to 1.69), 8.42% (6.16 to 10.74), 1.04% (0.68 to 1.41) and 6.69% (4.33 to 9.11) increase in cardiovascular admission, respectively; and 0.69% (95% CI: 0.18 to 1.21), 1.42% (0.98 to 1.85), 4.49% (2.22 to 6.80), 1.18% (0.79 to 1.57) and 7.69% (5.20 to 10.23) increase in respiratory admission, respectively. The elderly (≥65 years) seemed to be the most susceptible group to the effect of air pollution, whereas the effect estimate for male and female was not significantly different. Results from this study contributed the evidence to support the effects of air pollution (O3, NO2, SO2, PM10, and CO) on hospital admissions for cardiovascular and respiratory diseases, which might be useful for public health intervention in Thailand. Display omitted •Ambient air pollution was associated with increased risk of hospital admission in Bangkok.•Effect of air pollution on pneumonia, COPD, asthma, IHD, and MI admission was found.•Elderly is the most susceptible group to the effect of air pollution on respiratory admission.•First study to examine the effects of CO and NO2 on hospital admission in Thailand•NO2 had consistent effects on cardiovascular and respiratory hospital admission in Bangkok.