We have performed high field magnetization and multi-frequency electron spin resonance (ESR) measurements on single crystal and powder samples of Na2Fe2(C2O4)3(H 2O)2, abbreviated as SIO. The Fe2+ ions in SIO are bridged by oxalate groups to make two-leg ladders. The temperature dependence of magnetic susceptibilities parallel and perpendicular to the leg direction show very different and anisotropic behavior. Experimental results of the isomorphous compound Na2Co2(C2O 4)3(H2O)2 (SCO) are well explained by isolated magnetic dimers with an Ising-type anisotropy. Therefore, we analyze the frequency dependence of the ESR resonance fields by the same model with a fictitious spin 1 as in SCO. The agreement between experiment and calculation is satisfactorily good. Magnetic susceptibility and magnetization curves calculated with the same parameters obtained in the ESR analyses, however, do not agree well with the experimental ones. This indicates the presence of the exchange interaction between the dimers, namely the interaction along the leg.