  Temperature and velocity oscillations have been found in a rectangular
Rayleigh-B\'{e}nard convection cell, in which one large scale convection roll
exists. At $Ra=8.9 \times 10^{11}$ and $Pr=4$, temperature oscillation can be
observed in most part of the system and the oscillation period remains almost
constant, $t_T = 74 \pm 2$ seconds. Velocity oscillation can only be found in
its horizontal component, $v_y$ (perpendicular to large scale circulation
plane), near the cell sidewall, its oscillation period is also constant, $t_T =
65 \pm 2$ seconds, at these positions. Temperature and velocity oscillations
have different Ra dependence, which are respectively indicated by Peclect
number $Pe_T = 0.55Ra^{0.47}$ and $Pe_v = 0.28Ra^{0.50}$ . In comparison to the
case of cylindrical cell, we find that the velocity oscillation is affected by
the system geometry.
