In order to observe the differences in fixability and stability of the acetabular cup through the differences in cup size in relation to the size of acetabular reaming, we analyzed 75 cases of porous coated acetabular cup of Harris-Galante type which be followed up more that two years. All cases were divided into two group ; the first group (On line fit :45 cases) used with acetabular cup equal in size to the last reaming size and the second group (Press fit : 30 cases) used with acetabular cup 2mm larger than the last reaming size. For initial stability, all cases were fixed with acetabular screws. Both groups were analyzed for presence of gap, change of acetabular inclination, presence of radiolucent line, degree of displacement of vertical distance, stability and clinical results. The results were as follows; 1. The average Harris hip score was improved from 55.8 points preoperatively to 90.2 points postoperatively in the first group, from 56.9 points to 91.2 points in the second group, Clinical results are excellent in 39 cases, good in 4 cases, fair in 2 cases in the first group, and excellent in 29 cases and good in 1 case in the second group. 2. The gap between the acetabular cup and acetabulum was found initially at 17 zones in the first group but no gap at final follow up. In the second group, the gap was found at 23 zones and finally remained less that 1 mm gap was found at 3 cases in zone B1. 3. No radiologic instability of acetabular cup by Martell evaluation was noted in both 1st and 2nd group. Our data suggests that initial gap between acetabular cup and acetabulum in microporous coated acetabular cup were replaced with new bone and also, radiologic stability and good clinical results could be obtained in the second group used with acetabular cup 2 mm larger than the last reaming size.