In order to determine ways of improving reproductive efficiency 428 heads of Cheju ponies were investigated. The results are as follows; 1. The total occurrence rate of reproductive disorders for Cheju mares is 41.2%. 24.5% of those investigated showed every other year foaling, 16.7% showed other types of reproductive disorders. 2. The total reproductive disorders of ovarian and uterine diseases were 34 heads(10.9%), and 2 heads(0.6%) respectively. 3. Reproductive disorders of ovarian diseases was 89.5%. Those subdivided into ovarian dysfunction(34.2%), persistent corpus luteum(23.7%), ovarian atrophy(15.8%) and ovarian cyst, delayed ovulation(0.7%). respectively. 4. The dilivery disorders of abortion, dystocia and still-birth were 2.9%. 0.3% and 0.6%. respectively. 5. There were no significant differences in R.B.C, W.B.C, Hb. globulin between pregnant and non-pregnant mares, but albumin phosphorus, glucose in pregnant mares were of higher levels than non-pregnant mares. Generally albumin, Ca, glucose were of lower levels than those reported in other research.