The objectives of this study therefore is to ascertain the effect of mandatory IFRS on stock prices of manufacturing firmslisted on the Nigerian stock market and to determine whether there is a significant difference in stock prices given the mandatoryadoption of IFRS by firms in 2012. The study adopted the ex-post facto research design as it relied on historic data obtained fromthe website of Nigerian Stock Exchange from 2010 - 2015. The period from 2010 to 2015 include two periods of pre-adoption period(2010 - 2012) and post - adoption period (2013 - 2015). The study used firm-level data bothering on stock prices gathered from thewebsite of the Nigerian Stock Exchange. Stock prices data exist in daily, weekly and monthly forms. This study adopts the monthlystock prices and using the December month as the closing price of the stocks in each of the periods. All the manufacturing firmslisted on the Nigerian Stock Exchange formed the population of this study while the sample constitutes of twenty one (21) firmsclassified under the Consumer Goods industry sector of the Nigerian Stock Exchange. All variables for the sampled firms areobserved for each cross-section and each time period, resulting in a total number of one hundred and twenty six (126) observations.The study used a linear regression equation as well as the parametric paired samples t-test model structured in a way to enhancecomparisons of stock prices between the pre-IFRS and post-IFRS periods. Findings from the study therefore implies that themandatory adoption of IFRS has a positive and significant effect on stock prices of manufacturing firms listed on the Nigerian StockExchange and that there is a significant difference in stock prices of manufacturing firms given the mandatory adoption of IFRS in2012.