Pavement functional and structural parameters are important for pavement evaluation. This research aims at developing a prediction model to predict the deflection of flexible pavements using International Roughness Index (IRI) values to optimize maintenance activities without conducting deflection testing. For this purpose, Multan-Lodhran south bound section, a part of N-5 is selected as study area. It starts from km 929+000 and ends at km 867+000, having a length of 62 km, dual carriageway having two lanes each side. The data used for this study had been collected from the database of Road Asset Management Division (RAMD), National Highway Authority, Islamabad, Pakistan on basis of data availability and reliability. Collected data includes International Roughness Index (IRI) values measured by profilometer and deflection data measured by Falling Weight Deflectometer (FWD). Existing pavement structure data was collected from NHA Regional office. Linear and Logarithmic Regression analysis methodology using SPSS Statistical Analysis Software was used to develop correlation between pavement functional and structural parameters for flexible pavement. Also, Microsoft Excel was used for data analysis and validation purpose. The developed regression model showed significant relationship between deflection and IRI. Additionally, AASHTO Back calculation method was also used for determining the effective structure number (SNeff) using deflection results from FWD to determine correlation of IRI with another structural parameter. Considering this, a significant correlation was found between SNeff and IRI. However, it was found that other parameters i.e., subgrade resilient modulus (Mr) and pavement elastic modulous (Ep) did not show any significant relation. The result showed that if pavement IRI values are known, deflection and SNeff can be predicted providing feasibility for transportation agencies that do not have the capabilities as deflection tests are costly and require sophistication for its data handling.