Horses are non-ruminant and monogastric animal. If concentrates are fed plentifully at a time, undigested starch reaches large intestine and it activates amyloytic bacteria. Finally, it lowers a pH level and causes colic. Therefore, standard feed of concentrates and period of feed are important factors to fat up horses. This study was conducted to evaluate the productivity and carcass quality, quantity grade for Jeju horse (24 month old) with different fattening periods (4-, 8- and 13.5-month). The body weights were increased as the fattening periods increased and the increase rate was highest at 4-month group. The daily weight gain was decreased as the fattening periods increased such as 0.46 kg for 4-month group, 0.39 kg for 8-month group and 0.34 kg for 13.5-month groups. The daily intake was increased as the fattening periods increased such as 4.42 kg for 4-month group, 5.00 kg for 8-month group and 5.26 kg for 13.5-month groups. The daily feed conversion was increased as the fattening periods increased such as 11.71% for 4-month group, 17.69% for 8-month group and 18.53% for 13.5-month group. The backfat thicknesses and loin-eye areas were 3.50 mm, 83.50 ㎠ and increased to 4.20 mm, 94.00 ㎠ until they were fattening up for 8-month then they were decreased to 4.00 mm, 89.40 ㎠ when they were fattening up to 13.5-month. There was no significant difference in the meat color among 3 fattening groups, however, the a-values (redness) of meat color and b-values of fat color were highest (5.0 and 3.80, respectively) for 13.5-month group. The texture was not significantly different among 3 fattening groups. The maturity levels were in the 3.8∼4.5. All horses used in the experiment had the quality grade 2 and the horses for 13.5-month group had the quantity grade A, whereas the horses for 4-month group had the quantity grade B or C. Jeju horse had the low quality grade (quality grade 2) although the carcass yield increased when they were fattened from 24 month to 37.5 month.