In exercise prescription and training, a simple test that accurately evaluates abdominal strength without the use of a special device is required. This study aimed to examine the relationships between sit-ups and abdominal flexion strength tests and between both the above tests and the thickness of each abdominal muscle. Subjects comprised 13 healthy young males (age: 18.9 ± 0.64 yr, height: 170.2 ±0.70 cm, and weight: 64.9 ±8.80 kg). Sit-ups and abdominal flexion strength tests as well as muscle thickness measurements of rectus abdominis, external oblique, internal oblique, and transversus abdominis muscles were performed. The relationships between the measured values were examined using Pearson’s correlation coefficient. A significant correlation (r = 0.75) was found between sit-ups and abdominal flexion strength tests. In addition, both tests showed significant correlations (r > 0.65) with thicknesses of the rectus abdominis and internal oblique muscles. From the present results, the relationship between sit-ups and abdominal flexion strength tests is very strong, and both tests are related to muscle thicknesses of rectus abdominis and internal oblique muscles. The sit-ups test can evaluate not only muscle endurance of abdomen but also static strength.