The sawtooth activity in an ohmically heated plasma on the HT-7 tokamak has been investigated experimentally using soft x-ray (SXR) diode arrays, magnetic probes and other diagnostics. Its behaviour and occurrence are correlated closely to the discharge conditions such as the electron density $N_{e}$, the electron temperature $T_{e}$, the safety factor $q_a$ on the plasma boundary and the wall condition, etc. Generally speaking, as $N_{e}$ increases, the sawtooth activity grows, becoming stronger with a longer period $T_{s}$, a larger inversion radius $r_{s}$ and a larger amplitude fluctuation $\Delta I_{sx}/\bar{I}_{sx}$ on the SXR radiation intensity signal, and evolve from a small to a moderate sawtooth, to a large or even a giant sawtooth. Sometimes, a double sawtooth and a monster sawtooth activity emerge without apparent deterioration of the plasma confinement and without major plasma disruption. During the events, abundant magnetohydrodynamic phenomena are observed, including a long partial sawtooth collapse. In this paper, the general sawtooth behaviour and the dependence of the occurrence of the sawtooth instability on the electron density $N_{e}$ and wall condition have been summarized. Experimental properties of two types of double sawtooth and monster sawtooth activities are also presented and discussed. The set up of the HT-7 tokamak and the main diagnostics available are also briefly described.