Introduction: The electrocardiogram is used to assess the heart’s condition. Patients who come with symptoms of acute coronary syndrome must have the ECG immediately and the readings interpreted within 10 minutes, since any delays in giving treatment could result in significant tissue damage to the heart muscle. The objective was to determine the level of knowledge and practices regarding electrocardiogram (ECG) interpretation on cardiac arrhyth- mia among registered nurses working in critical care units. Method: A cross-sectional study was conducted among nurses at the critical care unit of Hospital Raja Permaisuri Bainun Ipoh, Perak. 105 respondents were recruited by convenience sampling. Respondents were given questionnaire on Knowledge and Practice on ECG interpretation. Results: this study found that 97.1% of the respondents have a good level of knowledge, while 76.2% have a good practice level regarding electrocardiogram interpretation on cardiac dysrhythmia. There were statistically significant differences in the knowledge level of ECG interpretation between the units in the hospital (χ2(3) = 33.962, p = 0.022) and education level (χ2(3) = 9.608, p < 0.001). There were statistically significant differences in the practice level of ECG interpretation between the years of working experience as a nurse (χ2(3) =13.726, p < 0.003) and working unit (χ2(3) = 21.887, p < 0.001). Conclusion: the level of knowledge regarding ECG interpretation on cardiac arrhythmia is generally good. Level of practice can be improved by the development of continuous education on ECG. Early detection of dysrhythmia is a crucial skill among nurses to give a prompt treatment.