【Abstract】 Objective To determine and analyze variation characteristics of heart rate (HR) and electrocardiogram (ECG) parameters in adult beagles within 24h. Methods The electrocardiogram was recorded by RM6240 multi-channel physiological signal acquisition decency via standard limb lead II in 12 adult beagle dogs which were under conscious state from 8:00 am to 8:00 am the next day. Results HR of beagle dogs showed a significant circadian rhythm with the fastest at 08:00 and the lowest at 12:00-14:00, HR at 08:00-09:00 was similar with that at 20:00-00:00. Corresponding with circadian rhythm of HR, P-R interval at 12:00 and S-T and Q-T interval at 11:00 was significantly increased, compared with that at 08:00 and 00:00 (p<0.05). Correlation analysis showed that there was remarkable correlation between HR and P-R, S-T and Q-T interval (P<0.01).Conclusion HR and part of ECG parameters in non-anesthetized adult beagle dogs showed a significant circadian rhythm, HR was slowly in the day, while P-R, S-T and Q-T interval prolonged correspondingly .