Objective To observe the change features of ECG in navy personnel onboard large and small vessels after they re -turn from prolonged deployment to the Gulf of Aden .Methods One hundred and forty navy personnel onboard the auxiliary ship ( sim-ply designated as the large vessel ) and 79 people onboard the escort ( simply called the small vessel ) respectively received routine body surface 12-lead ECG detection , on the day they returned to the homeport , following prolonged deployment at sea .Abnormality in ECG and changes in heart rate and wave intervals were compared between the 2 ship crews.Results The incidence of premature ventricular contraction for the afloat personnel onboard the small vessel was higher than that for those onboard the large vessel (P<0.05).No sta-tistical differences could be noted in other indicators , such as heart rate, left ventricular high voltage , ST-T change, early repolariza-tion, intermittent one or two degree type I atrioventricular block , as well as waveform interval (P>0.05).Conclusion The incidence of arrhythmia for the afloat personnel onboard small vessel was evidently higher than that for the afloat personnel onboard the large ves -sel, which might be related with autonomic nervous system dysfunction and increased sympathetic nerve excitability .