  The purpose of this work is to search the Yangbajing neutron monitor data
obtained between 1998 October and 2000 June for solar neutrons associated with
solar flares. Using the onset times of 166 BATSE-detected flares with the GOES
peak flux (1 -- 8 \AA) higher than $1.0 \times 10^{-5}$ $\mathrm{Wm^{-2}}$, we
prepare for each flare a light curve of the Yangbajing neutron monitor,
spanning $\pm$ 1.5 hours from the BATSE onset time. Based on the light curves,
a systematic search for solar neutrons in energies above 100 MeV from the 166
flares was performed. No statistically significant signals due to solar
neutrons were found in the present work. Therefore, we put upper limits on the
$>$ 100 MeV solar-neutron flux for 18 events consisting of 2 X and 16 M class
flares. The calculation assumed a power-law shaped neutron energy spectrum and
three types of neutron emission profiles at the Sun. Compared with the other
positive neutron detections associated with X-class flares, typical 95%
confidence level upper limits for the two X-class flares are found to be
comparable to the lowest and second lowest neutron fluxes at the top of the
atmosphere.In addition, the upper limits for M-class flares scatter in the
range of $10^{-2}$ to 1 neutrons $\mathrm{cm^{-2}s^{-1}}$. This provides the
first upper limits on the solar-neutron flux from M-class solar flares, using
space observatories as well as ground-based neutron monitors.
