Electron beam irradiation into type-Ib diamond is known as a good method for the creation of high concentration negatively-charged nitrogen-vacancy (NV⁻) centers by which highly sensitive quantum sensors can be fabricated. In order to understand the creation mechanism of NV⁻ centers, we study the behavior of substitutional isolated nitrogen (P1 centers) and NV⁻ centers in type-Ib diamond, with an initial P1 concentration of 40–80 ppm by electron beam irradiation up to 8.0 × 10¹⁸ electrons/cm². P1 concentration and NV⁻ concentration were measured using electron spin resonance and photoluminescence measurements. P1 center count decreases with increasing irradiation fluence up to 8.0 × 10¹⁸ electrons/cm². The rate of decrease in P1 is slightly lower at irradiation fluence above 4.0 × 10¹⁸ electrons/cm² especially for samples of low initial P1 concentration. Comparing concentration of P1 centers with that of NV⁻ centers, it suggests that a part of P1 centers plays a role in the formation of other defects. The usefulness of electron beam irradiation to type-Ib diamonds was confirmed by the resultant conversion efficiency from P1 to NV⁻ center around 12–19%.