Purpose: The aim of the study was the evaluation of the anal sphincter integrity and the occurrence of anal incontinence after fistula in ano surgery using 3D endoanal ultrasonography. Methods: 61 patients with different types of fistula in ano were enrolled in the study during the period from October 2012 to December 2013. All the patients were subjected to preoperative and postoperative 2D and 3D endoanal ultrasonography (EAUS). Different techniques of fistula in ano surgery were done. Measurements of the anal sphincters length were calculated preoperative and postoperative. Results: The patients' age ranges from 19 to 74 years. 36 (60%) of the patients were males. 20 (32.8%) patients had low transsphincteric fistula, 18 (29.5%) patients had intersphincteric fistula, 10 (16.4%) patients had extrasphincteric fistula, 7 (11.5%) patients had suprasphincteric fistula, 5 (8.2%) patients had high transsphincteric fistula and 1 (1.6) patient had submucosal tract. It was observed that the highest percent of divided external anal sphincter (EAS) and internal anal sphincter (IAS) were 75.7% and 87.3% respectively. 4 cases were incontinent. The lowest percent of the divided external anal sphincter (EAS) and internal anal sphincter (IAS) in incontinent patients were 63.5% and 44.4% respectively that showed statistical significance (P value is 0.001, 0.002 respectively). Conclusion: During fistula in ano surgery preservation of the anal sphincter is an integral part of the techniques. Inspite of the small number of the cases and the anal incontinence is a multifactorial disease, the patient is at risk of incontinence if >63 % of external anal sphincter (EAS) and 44% of internal anal sphincter (IAS) were divided during surgery. Key words: Fistula in ano, external anal sphincter, internal anal sphincter and endoanal ultrasonography. This study helped us not to dived more than 60% or 40% of the EAS and IAS respectively during fistula in ano surgery.