The effects of seedling age on uniformity in growth, rootability and adaptability to automatic plug transplanter were investigated in cabbage plug seedlings for their timely transplantation. Growth of plug seedlings and fresh weight of cabbage head were recorded after transplanting. Major findings:1. There was a positive corelation between the seedlings age and their uniformity in growth in nursery plug system.2. This uniformity in growth is contributed to the limited size of the plug for root growth.3. The seedlings with 2.5 leaves were inferior to any older seedlings in rootability.4. The age of transplanted seedlings had no effect on their subsequent growth.5. The number of leaves increased as seedlings aged in nursery plug system.6. The seedlings with 5.0 leaves were considered unsuitable for use in the automatic plug transplantes due to their large size and thier spreading shape.7. The head weight in cabbage was higher when older seedlings were used in transplanting.From these observations, taking all factors (including cost and management time in raising seedlings) into consideration, we conclude that 3.5 leaves aged plug seedlings were suitable for timely transplanting in automatic mechanical transplantation of cabbage cultivar Matsunami.