The need to identify the ‘real’ gifted children in Nigeria for proper placement into gifted schools and the operationalization of giftedness concept promoted the development and validation of Giftedness Assessment Instrument (GAI). Following the due psychometric process of item selection, GAI comprises three components derived from several sources in a manner that retained the merits and eliminated the demerits of the sources, hence it is said to be integrated. The three components are, Above average /cognitive ability (GAI-1), Creative ability (GAI-2) and Task Commitment/Motivation ability (GAI -3) with their independent attributes obtained through factor analysis using principal components with varimax rotations. The test was administered to 600 primary six pupils from public and private schools randomly selected from the six educational zones of Lagos State. The results obtained include: norms- males and females for public and private schools, high reliability and validity coefficients and discriminate validity with other standardized convergent and divergent measures of giftedness. To enhance the interpretation of the results, the scores were transformed into standard scores.