Indices concerning morphological characters of rice ( Oryza sativa L.) roots were determined by means of root length estimated with a root length scanner (an applied instrument of the line intersection method). Prior to actual root measurements, nylon thread with different diameters (0.07, 0.10 and 0.33mm) was measured with the root scanner in order to check its performance. It was found that the measurements were exact when the diameters of nylon thread were not less than 0.lmm. Thus, the instrument was thought to be capable of detecting all of the primary roots and most of the 'thick' secondary roots in rice plants. Root measurements were conducted for some paddy rice cultivars with respect to primary root length (PRL) and FAA-insoluble structural dry matter weight ('DW') as well as root length estimated with the root scanner (ERL). To indicate root branching and root thickness (diameter), we proposed the 'coefficient of branching' and the 'coefficient of thickness' which were defined as ERL/PRL ratio and 'DW'/ERL ratio, respectively. In this study, the former ranged from 2.2 to 4.6 (m/m), and the latter ranged from 9.18 to 10.81 (mg/m). After analyzing differences between plant stages or cultivars, these 'coefficients' were thought to be useful indices for relative comparisons.