BACKGROUND: Osteoarthritis of the knee is the most common chronic joint disease that involves middle aged and elderly people. The purpose of this study was to investigate the effect of aromatherapy massage with lavender essential oil on pain in patients with osteoarthritis of the knee. METHODS: In this single-blinded, randomized clinical trial, 90 patients with osteoarthritis of the knee who referred to the outpatient rheumatology clinics affiliated with Birjand University of Medical Sciences were selected through convenience sampling method. They were randomly assigned to three groups: intervention (aromatherapy massage with lavender essential oil), placebo (massage with almond oil) and control (without massage). The patients were evaluated at baseline, immediately after the intervention, 1 week, and 4 weeks after the intervention in terms of pain via visual analogue scale. The data were analyzed in SPSS (version 16) using the repeated measure ANOVA, one-way ANOVA, and chi-squared test. RESULTS: Pain severity of the patients in the intervention group was significantly different immediately and 1 week after the intervention compared with their initial status (p < 0.001) and that of the control group (p < 0.001 and p = 0.009 respectively). However, at the third phase of follow-up (i.e., 4 weeks after the intervention), there was no significant difference between the groups according to the visual analogue scale (p = 0.67). CONCLUSION: Aromatherapy massage with lavender essential oil was found effective in relieving pain in patients with knee osteoarthritis. However, further studies are needed to confirm findings of this study.