This study was undertaken using a total of 2,577 racing records of horses from September, 1989 to July, 1990 that collected at the K. H. A. A. (Korean Horse Affairs Association) to investigate effects of environmental factors and to estimate genetic variance for racing performance. Results obtained from this study were as follows. Horse`s age, racing number, track condition and racing season showed significant effects on racing time, and winner`s racing time was not appropriate environmental factor for mean racing time in Korea. Racing performance was the best when horse`s age was 3-4yr, racing number was l situated inside, moisture content was more than 16% in track and racing seasons were spring and winter. Estimates of σ²_e by Maximum Likelihood were smaller than those by Minimum Variance Quadratic Unbiased Estimation (MIVQUE). Adjustment for winner`s racing time reduced estimates of σ²_s, σ²_h, and σ²_e, especially, estimates of σ²_h, which were reduced by an average of about 1000% and estimates of repeatabilities were lower than those of heritabilities. Estimates of heritabilities and repeatabilities by nationality ranged from .13±.17 to .30±.20 and from .36±.07 to .52±.14, respectively. Pooled estimates of heritabilities and repeatabilities were .19±.13 and .42± .06 respectively.