A novel rhodamine based probe has been synthesized as an “off–on” chemosensor for Fe 3+ . Upon coordination with Fe 3+ , the probe displayed good brightness and fluorescent enhancement. A linear relationship was observed to exist between the relative fluorescence intensity of this probe and the concentration of Fe 3+ in the range of 5 μM–20 μM with a detection limit of 5 μM. It offered highly sensitive toward Fe 3+ over other ions. The recognizing behaviors toward Fe 3+ have been investigated both experimentally and computationally. It can be expected that Fe 3+ coordinated with the N atom of thiazole moiety in the probe accompanied by the transferring of electrons of the phenylthiazole resulted in the opening of the spiro-ring.