  We present a detailed \chandra study of the galaxy group NGC 1550. For its
temperature (1.37$\pm$0.01 keV) and velocity dispersion ($\sim$ 300 km
s$^{-1}$), the NGC 1550 group is one of the most luminous known galaxy groups
(L$_{\rm bol}$ = 1.65$\times10^{43}$ erg s$^{-1}$ within 200 kpc, or 0.2 \rv).
We find that within $\sim 60$ kpc, where the gas cooling time is less than a
Hubble time, the gas temperature decreases continuously toward the center,
implying the existence of a cooling core. The temperature also declines beyond
$\sim$ 100 kpc (or 0.1 \rv). There is a remarkable similarity of the
temperature profile of NGC 1550 with those of two other 1 keV groups with
accurate temperature determination. The temperature begins to decline at 0.07 -
0.1 \rv, while in hot clusters the decline begins at or beyond 0.2 \rv. Thus,
there are at least some 1 keV groups that have significantly different
temperature profiles from those of hot clusters, which may reflect the role of
non-gravitational processes in ICM/IGM evolution. NGC 1550 has no isentropic
core in its entropy profile, in contrast to the predictions of `entropy-floor'
simulations. We compare the scaled entropy profiles of three 1 keV groups
(including NGC 1550) and three 2 - 3 keV groups. The scaled entropy profiles of
1 keV groups show much larger scatter than those of hotter systems, which
implies varied pre-heating levels. We also discuss the mass content of the NGC
1550 group and the abundance profile of heavy elements.
