Purpose: This study investigated knowledge, attitudes, and nursing stress related to lifesustaining treatment among oncology nurses. Methods: A descriptive study design was used. Data were collected through a survey from April 1 to May 31, 2022. The participants were 132 nurses working in the oncology ward of a tertiary hospital in Seoul. Data were analyzed using the SPSS 25.0 program with descriptive statics, the independent t-test, analysis of variance, and Pearson correlation coefficients. Results: The average scores for knowledge, attitudes, and nursing stress related to life-sustaining treatment were 14.42, 3.29, and 3.96, respectively. Significant differences in knowledge about life-sustaining treatment were observed based on clinical experience (P=0.029) and education about life-sustaining treatment (P=0.044). Attitudes toward life-sustaining treatment varied significantly with education about life-sustaining treatment (P=0.014), while stress levels differed significantly across working units (P=0.004). A positive correlation was found between the dilemma of extending or stopping life-sustaining treatment (a subdomain of nursing stress) and attitudes toward life-sustaining treatment (r=0.260, P=0.003). Conclusion There was no significant correlation between the nursing stress experienced by oncology nurses and their knowledge and attitudes toward life-sustaining treatment. However, a more positive experience with life-sustaining treatment education was associated with higher stress levels related to the dilemma of extending or stopping life-sustaining treatment. Therefore, it is crucial to develop strategies to manage this dilemma and reduce stress in the field.