This paper applies the adjusted balance of payment (BOP) constrained growth framework modified by Thirwall and Hussain (1982) on Nigeria's economic growth to estimate the determinants of the long run rate of growth in Nigeria. With Nigeria adopting the import substitution industrialization policy in 1960, we apply cointegration test on time series data to estimate the long-run relationship between Nigeria's real GDP (output) and its real export. Results signify cointegration between our variables, lending support to Thirwall's BOP constrained model as a suitable framework to explain Nigeria's long term growth and reinforces the opinion that external factors constrain Nigeria's economic growth.