Over the last thirty years many urban areas in Nigeria have experienced dramaticgrowth as a result of rapid urbanization and population growth. While cities inNigeria as well as other parts of the world, command a dominant role as bothcentres of production and consumption, rapid urban growth of Nigerian cities hasseriously outstripped the capacity of most cities to provide adequate basicservices to their citizens. This study investigates the factors that led to the rapidurbanization of Nigerian cities, especially Onitsha metropolis; the constraints toeffective provision of urban amenities; and the causes of poor urban planning.The findings showed that the presence of substantial social infrastructure inOnitsha urban area has been the major factor that engendered its urbanization. Italso revealed that there is rapid urbanization in relation to effective plans toprovide adequate urban infrastructures/amenities to service this trend.Furthermore, there is a strong correlation between lack of good populationprojection, inaccurate census data and poor urban planning in Onitsha. Hence,these engenderers the bad effects of urbanization. The study calls for a revision ofthe existing planning practice to take into account the growth in the urban area,and the consequent increase in the demand for urban infrastructural services. Itgave suggestions on ways to improve urban planning in Nigerian cities toinclude; proper articulation and implementation of planning policies andprogrammes to address future influx of people in Onitsha urban area; urbanrenewal and slum clearance programmes; imposition of a moratorium onresidential developments in urban areas and most importantly, governments’commitment to tackle the multifarious problems of urbanization and urbanplanning our urban areas.