We demonstrate an ultrabright narrow-band two-photon source at the 1.5 -mu m telecom wavelength for long-distance quantum communication. By utilizing a bow-tie cavity, we obtain a cavity enhancement factor of $4.06times 10^4$. Our measurement of the second-order correlation function $G^{(2)} ({tau})$ reveals that the linewidth of $2.4$ MHz has been hitherto unachieved in the 1.5 -mu m telecom band. This two-photon source is useful for obtaining a high absorption probability close to unity by quantum memories set inside quantum repeater nodes. Furthermore, to the best of our knowledge, the observed spectral brightness of $3.94times 10^5$ pairs/(s$cdot$MHz$cdot$mW) is also the highest reported over all wavelengths.
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