ISSR markers were used to determine the genetic variation and sex determination of the three life-stages of 41 samples of Gracilaria corticata from two regions of the Persian Gulf and Oman Sea namely Bostaneh and Lipar. The specimens were cultured in PES media to observe the different life stages. DNA was extracted by modified CTAB method. After screening of 20 ISSR primers, four primers were selected that produced clear reproducible fragments and were suitable to find sexual diversity for further analyses. The four primers generated 75 bands ranging from 250 to 3000 bp, corresponding to an average of 18.75 bands per primer. A set of four anchored primers amplified 75 bands out of which 100 % were polymorphic among 41 samples. PIC value ranged from 0.28 to 0.33 and marker index ranged from 4.48 to 6.51 per primer. The mean value of Shannon’s index was 0.45. By using primer "AB","ABC₁" were identified three life stages of this alga. Primers "A" and "C" were also able to detect diploid tetrasporophyte but haploid males and females was determined by "A" and "C" respectively. Gst value was 0.058, indicating that 83% of the genetic diversity resided within the population. Clustering analysis using WARD algorithm based on Nei's Unbiased Measures of Genetic distance, classified the G. corticata individuals into five major groups. The PCOA data confirmed the results of clustering. The results of this study reveal that ISSR markers could be used efficiently for genetic differentiation of G. corticata individuals in different regions