The effect of genetic grade (strain) on body weight and body linear measurements of the pure grade dual purpose French guinea fowl strain (116 birds) and their mixed genetic grade (32 birds) was determined using a linear mathematical model. Birds were raised under intensive system in Katsina state. Loss in vigour of the pure grade dual purpose French guinea fowl was calculated by difference. There was a significant (P < 0.05) effect of strain (genetic grade) on body weight and body linear measurements. The pure grade dual purpose French guinea fowl cocks and hens had significantly higher body weight and most body linear measurements than counterpart dual purpose mixed genetic grade guinea fowl cocks and hens. The pure grade dual purpose French guinea fowl cocks and hens had lower coefficients of variation (7.76 and 9.60 %) than the dual purpose guinea fowl cocks and hen mixed genetic grades (13.11 and 13.54 %). The dual purpose mixed genetic grade guinea fowl cocks and hens had longer helmet length than counterpart pure grade dual purpose French guinea cocks and hens. Strain (genetic grade) had no effect on tail length of guinea hens. There were high loss of vigour (56.25 and 57.69 %) of helmet thickness followed by (28.82 and 37.38 %) for body weight of the dual purpose French guinea fowl cocks and hens. Body length (3.76 and 0.56 %) and thigh length (4.22 and 1.85 %) respectively for guinea fowl cocks and hens, were the morphometric traits that had the lowest reduction in vigour among all quantitative traits evaluated. The dual purpose French guinea fowl and their mixed genetic grade had higher body weight than counterpart local guinea fowl strains. Introduction of the dual purpose French guinea fowl into the tropics can aid the diversification of rural poultry production.