Abstract: Nutrient solution culture was employed to investigate the response of oat seedling roots to phosphorus deficiency. The results showed that after 30 days of treatment, the roots biomass of oat with P deficiency was significantly lower than that of oat supplied with sufficient P, however, the total root length of P-deficiency oat was not significant lower than high-P oat’s. It also showed that root /shoot ratio and specific root length for P-deficiency oat were higher than for high-P oat. Smaller root diameter for P-deficiency oat root axes was coincident with the higher specific root length, so it can be the reason for higher specific root length of P-deficiency oat. It is well known greater specific root length implies the root system exploring more soil for P with same assimilates consumption, therefore, the decrease of root diameter and increase of specific root length can be thought as the adaptive response of oat seedling to P deficiency.