Purpose:  To compare the mean axial length measured by acoustic biometry with optical biometry. Design:  Descriptive observational study. Place and Duration of Study:  Department of Ophthalmology Liaquat National Hospital and Medical College, Karachi from November 2018 to April 2019. Methods:   were 246 patients with visually significant cataract who were recruited in this study by consecutive sampling. Axial lengths were measured by non-contact optical method using Carl Zeiss IOL master. After instillation of local anesthetic, axial lengths were re-measured by contact method with A-Scan probe. All readings were taken by a single observer to avoid data collection bias. Mean and standard deviation was computed for quantitative variable i.e. age, axial length by Applanation ultra sound and axial length by optical biometry. Frequency and percentage was calculated for qualitative variables i.e. gender. Differences of axial length between the Applanation and optical biometry were compared by using pair T-test. Effect modifiers like age and gender were addressed through stratification, post stratification independent test for two groups and ANOVA for more than two groups was applied. Results:  The average age of the patients was 58.09 ± 7.27 years. Mean axial length by optical biometry was 23.744 ± 0.74mm as compared to 22.29 ± 0.76 mm by acoustic method and this difference was statistically significant (P = 0.0005). Conclusion:  Results of axial length with Optical and acoustic biometry are significantly different with p value less than 0.05. However, optical biometers fail in cases of dense media opacities where acoustic biometry in needed. Key Words:  Cataract, Axial length, Applanation ultrasound, Optical Biometry.