Background: Lateral internal sphincterotomy is the ideal and gold standard surgical management of chronic anal fissure. The extent of lateral internal sphincterotomy is still         a subject of discussion. Excess division of the internal sphincter leading to incontinence either to stool or flatus, so we must determine the extent of sphincterotomy. Subjects and methods: Between the period from February  2018 and October  2018 in the General Surgery Department of Zagazig University and El – Ahrar teaching Hospitals, 62 female patients with chronic anal fissure  with mean ages of (34.1 + 8.2) and (41.8 + 9.05) of group A and B respectively complaining of chronic anal fissure. Group A of (31) patients (50%) of study cases were subjected to lateral internal sphincterotomy up to the fissure apex. Group B of (31) patients (50%) of study cases were subjected to lateral internal sphincterotomy to less than 1 cm from anal verge. Results: Group A showed complete fissure healing in 4 weeks  (25) patients (80.6%) with     (6 )patients cases comprising (19.35%) with delayed wound healing in 6 weeks .( 9 ) patients ( 29 % ) had transient incontinence to flatus . Group B showed complete fissure healing in 5 weeks  (20) patients (64.5%) with (11 )patients cases comprising (35.5%) with delayed wound healing in 6 weeks .( 2) patients ( 6 % ) had transient incontinence to flatus Conclusions: In females with chronic anal fissure, division of internal sphincter must not exceed 1 cm from anal verge and not reach apex of the fissure to preserve the internal sphincter function and prevent incontinence either to stool or to flatus.