The accretion-induced pulse-period changes of the Be/X-ray binary pulsar X Persei were investigated over a period of 1996 January to 2017 September . This study utilized the monitoring data acquired with the RXTE/ASM in 1.5–12 keV and MAXI/GSC in 2–20 keV . The source intensity changed by a factor of 5–6 over this period . The pulsar was spinning down for 1996-2003 , and has been spinning up since 2003 , as already reported . The spin up/down rate and the 3–12 keV flux , determined every 250 d , showed a clear negative correlation , which can be successfully explained by the accretion torque model proposed by \citet GL79 . When the mass , radius and distance of the neutron star are allowed to vary over a range of 1.0–2.4 solar masses , 9.5–15 km , and 0.77–0.85 kpc , respectively , the magnetic field strength of B = ( 4 - 25 ) \times 10 ^ { 13 } G gave the best fits to the observation . In contrast , the observed results can not be explained by the values of B \sim 10 ^ { 12 } G previously suggested for X Persei , as long as the mass , radius , and distance are required to take reasonable values . Assuming a distance of 0.81 \pm 0.04 kpc as indicated by optical astrometry , the mass of the neutron star is estimated as M = 2.03 \pm 0.17 solar masses .