Abstract: Root excretion of oxygen is an important feature of Sagittaria trifolia. For better understanding the characteristics of the radial oxygen loss (ROL) from roots of the plant in natural sediment, high-precision dissolved oxygen microelectrodes were used to measure penetration depth and distribution of oxygen under different light intensities (dark and light). Substantial difference in oxygen distribution was observed between parts of the roots. Thickness of the oxygen diffusion layer in the rhizosphere under light and in dark varied along the root in the following order: at 1/2 of the root length (0.98 and 0.72 mm) > at 3/4 (0.68 and 0.28 mm) > at root tip (0.58 and 0.44 mm) > at 1/4 of the root (0.42 and 0.32 mm). Content of dissolved oxygen on root surface under light varied along the root in the following order: at 1/2 of the root length (64.56% of air saturation) > at 3/4 (52.73%) > at root tip (38.55%) > at 1/4 (20.55%), which was related to barrier to ROL, development of aerenchyma, and metabolism and respiration of the tissue. ROL from the roots of S. trifolia, no matter under light or in dark, occurs all the time, and is significantly affected by light intensity. Both the content of dissolved oxygen on root surface and the thickness of the oxygen diffusion layer in the rhizosphere are higher under light than in dark. Among all the measuring points, the point at 1/2 of the root length had the highest ROL in both light conditions (P<0.05). Under light significant differences were found between all the other points in content of dissolved oxygen, but in dark the contents of dissolved oxygen at these points tended to be on the same level (P>0.05).