A model is proposed to analyze root system morphology using simulated root length densities. A crop population with fibrous root systems is imagined, and three simple assumptions are set : (1) uniform elongation of primary roots, (2) homogeneous branching and (3) hemispheric rooting zone. The assumptions give mathematical descriptions of root length densities (p) so that the variable p can be expressed as a mathematical function of the position in the soil. Simulation of. p is conducted with a computer program which includes five parameters : distance between hills (a), distance between rows (b), hill size ('hill radius' ; ro), size of a rooting zone ('rooting zone radius' ; rmax) and degree of root branching ('root length density constant' ; k). The values of k and rmax were evaluated so that simulated root length densities (pmodel) fit actual densities (pactual). Indices of root system morphology were algebraically derived from the simulation parameters. The indices correspond to important characters that are difficult to be estimated from usual field studies. Part II will cite examples of analysis on rice (Oryza sativa L.) root systems using this model.