It is well known that water is one of the most important chemical constituent in the nature, and thus is regarded as the genesis of life. Meanwhile, water plays an irreplaceable role in biology process, such as regulating the affinity between fatty acids and alkyl chains and controlling the conformation of DNA. In the crystal chemistry, water molecules in compounds will also significantly affect its structure, physical and chemical properties. Among them, single crystal to single crystal (SCSC) transformations induced by water molecules have attracted great attention, because those transformations often change the physical properties such as magnetic, dielectric and proton conduction. More importantly, it is facile to obtain detailed structural information, which leads to more directly and better understanding the transformation mechanism and structure-properties relationship. In this work, we report the SCSC transformation in the compound of {(Me₂NH₂)₂- [Fe₂(ox)₂Cl₄]•H₂O} _n (ox=oxalate) ( 1•H₂O), which was prepared through the hydrothermal reaction of oxalate and FeCl₃ in the presence of dimethylamine chloride. Under vacuum and heating conditions, 1•H₂O underwent SCSC transformations, formed compound 1. 1•H₂O and 1 were characterized by X-ray single-crystal diffraction, powder X-ray diffraction, magnetic, dielectric and thermogravimetric analysis. Magnetic studies on 1•H₂O and 1 show that the presence or absence of guest water molecules had no significant effect on their magnetic interaction. Investigation on the dielectric properties of 1•H₂O and 1 reveals that the presence of guest water molecules can influence the dynamic relaxation process of guest amine and water in the compound. Compound 1•H₂O exhibited three dielectric relaxations at 250 K, 80 K and 50 K respectively, while 1 only showed a single relaxation process at 80 K. This work sheds light on the further exploration of SCSC transformations associated with the dielectric relaxation.