This study focused on the estimation of genetic relationships between weekly live weights and sexual maturity of male and female Japanese quail. The live body weight data of a laying-type quail line over time were collected from hatching to six weeks of age. Sexual maturity was determined as the day of production of cloacal gland foam for males, and the day of laying the first egg for females. (Co)Variance components and genetic parameters were estimated, using MTDFREML, a restricted maximum likelihood (REML) procedure using a model with direct and maternal genetic effects. Heritability estimates of weekly live weights ranged from moderate to high (0.20 – 0.60), and were higher for males than for females. The variance due to maternal effects disappeared gradually for males but rapidly for females as the chicks grew older. Heritability of age of sexual maturation was 0.24 ± 0.008 and 033 ± 0.136 for males and females, respectively. Genetic correlations between ages of reaching sexual maturity and live weights (except hatchling weight) were favourable for males (-0.16 to -0.45) and unfavourable for females (0.14 to 0.22). Genetic correlations between males and females ranged from moderate (0.46 ± 0.083) to high (0.98 ± 0.139) for weekly live weights, and were 0.71 ± 0.24 for age of reaching sexual maturity. Sexual size dimorphism can be explained by the differences in pattern of heritability and correlation estimates between reaching sexual maturity and live weights for males and females.