Recently nitrogen(N) deposition has been increasing due to anthropogenic activities. N deposition may work as a nutrient when the soil is infertile. However, long-term N deposition may cause an imbalance between N and other nutrients such as phosphorus (P). P is also an essential nutrient for plant growth, however, as P is frequently immobile, plants must efficiently uptake P by producing new fine roots. However, knowledge on how different N and/or P conditions affect the root growth of hybrid larch (F1) is still limited. We examined the effects of N loading under low and high P conditions on F1, which is an important afforestation species in Hokkaido. Needle N: P ratio, above and below-ground growth and relative fine root production were measured. Needle N: P ratio was higher under low P loading, and aboveground growth of seedlings increased with N loading at both P conditions. Relative fine root production was decreased by N loading. On the other hand, fine root to total dry proportion was increased by N loading at low P addition. It can be concluded that N loading has different effects on above- and below-ground growth of F1 and its effects may also differ according to P conditions.