Study aim: The purpose of this study was to investigate the effect of Nigella sativa on kidney stone dissolving. Design: In this research, 60 eligible patients with a history of kidney stone equal or larger than to 5 mm diameter on kidney ultrasonography were randomly divided into two groups of control and intervention. Group allocation was concealed by assigning a unique code to each participant. Settings and conduct: We designed a randomized, double arm, double blind, placebo-controlled clinical trial. This study was carried out in urology clinic of Ghaem and Imam Reza Hospital, affiliated to Mashhad University of Medical Sciences, Iran, from November 2015 to May 2016. Participants/Inclusion and exclusion criteria: Inclusion criteria : Men and women aged 20 to 60 years, people 20-60 years old with kidney stone disease that has been confirmed by ultrasound sonography. They should have kidney stones equal or larger than 5 mm. Exclusion criteria : Breast feeding, pregnancy, any renal disorder which requires new treatment protocols and known allergy to black seed. Intervention groups: Eligible patients were divided into two groups. Patients were randomly assigned to receive either a 10 weeks black seed capsules (500 mg 2 times per day) as the intervention group, or placebo capsules (500 mg 2 times per day) as the control group. Main outcome variables: The primary outcome measure in this trial was change in the size of renal stones by using ultrasound sonography. Secondary outcome measures were changes in the levels of serum calcium and urine PH.