Objective To study the effect of coffee on heart rate variability(HRV)in young adults in order to determine the effect of coffee ingestion on cardiac autonomic nervous system(ANS)activity.Methods There were sixteen healthy young nonhabitual caffeine consumers as subjects enrolled in this study(height 167cm±7.2cm，body mass 61.3kg±6.3kg，age 24.9years±2.8years).Coffee ingested by the subjects was 6 mg/kg caffeine as moderate dosage.And the dynamic electrocardiogram signals were recorded and analyzed with a dynamic Holter before and after coffee ingestion during 8:40 to 10:45 in the morning. HRV parameters included the mean standard deviation of all NN intervals，the root mean square of successive differences(rMSSD)，the percentage of successive NN intervals differing >50 ms(PNN50)，the normalized low frequency power(LFnorm)，the normalized high frequency power(HFnorm)，and the low frequency(LF)and high frequency(HF)ratio LF/HF.Results There were significant differences before and after taking coffee in HRV parameters and the mean heart rate(HR).HR，LFnorm，LF/HF decreased 7.5%，17% and 35%，respectively.rMSSD，PNN50 and HFnorm increased 82%，80% and 58%，respectively.HR and LF/HF changed more obviously over time after taking coffee.ΔLF was highly correlated with ΔHF and ΔLF/HF with the correlation coefficients of -0.980 and 0.903.Conclusions Coffee with moderate dosage caffeine can cause the change of HRV and the decrease of HR，inhibit sympathetic nervous system activity(SNSA)，and enhance parasympathetic nervous system activity(PNSA).