Purpose: This study aimed to determine the effectiveness of Relaxation Breathing Exercise (RBE) on fatigue in patients with gynaecological cancer undergoing chemotherapy.Methods: We used a quasi-experiment with pre-and post-test design.Forty-two patients were consecutively sampled,21 of whom were assigned to the group receiving RBE four times a day and 21 to the group receiving RBE twice a day.Fatigue scores were measured every day for 7 days for both groups using the Piper Fatigue Scale.Data obtained were analysed using repeated-ANOVA and independent ttest with a significant level of a < 0.01.Results: We found a significant decrease (P< 0.01) in the mean fatigue scores of both groups (four times RBE in a day group =3.29 ± 0.59 and twice RBE in a day group =4.19 ± 0.61) after the completion of the intervention.However,the four times RBE a day group showed larger decrease on fatigue scores than the twice RBE a day group did (Mean Difference =0.91;99% CI =0.41 to 1.41;P =0.001).Conclusion: Conducting RBE four times a day effectively alleviated fatigue better than RBE twice a day on gynaecological cancer patients undergoing chemotherapy.