An F2 population of 239 chickens was obtained by an intercross between Nagoya (NAG), a native Japanese breed with low growth, and White Plymouth Rock (WPR), a Western breed with high growth. Using SNP markers obtained by restriction site-associated DNA sequencing, genome-wide QTL analysis was performed and it revealed three QTL for early postnatal growth in the F2 population at genome-wide 5% significance levels. The most highly significant QTL affecting body weights at 2-4 weeks of age and weight gains at 2-3 and 0-4 weeks was located on GGA4 between 34.0 and 65.6 Mb with LOD scores of 3.9-5.9 and it explained 4.9-9.9% of the total variance of the traits. The analysis provided evidence for significant QTL on GGA2 between 105.6 and 125.2 Mb (LOD = 4.6) and on GGA1 between 51.1 and 61.6 Mb (LOD = 4.0) which had effects on body weight at 3 weeks and body weight gain at 0-1 week respectively. These two genomic regions explained 6.6 and 6.9% of the phenotypic F2 variance of the corresponding traits respectively. The allele derived from WPR at all QTL increased the corresponding traits. Neither sex-specific nor epistatic QTL was detected. The results showed that the GGA4 QTL affecting multiple traits is a key locus responsible for early growth in our chicken cross, suggesting that this QTL may make a great contribution to genetic improvement of growth performance of the NAG breed with a low growth rate.