In the present research, amounting of the trace little amount of Cobalt (II) ions in aqueos Samples by using modified disks of octadecyl silica membrane by nano-Fe3O4-encapsulated-dioctyl phthalate and linked-diethylenetriamine (magnetic nano- Fe3O4 -DOP- DTA)which in order to improve the performance of extraction and preconcentration was used. Amounting of the ions density in solution was performed by the method of flame atomic absorption spectrometry. The effective parameters on extraction such as: effect of pH, the amount and eluent solvent, time of extraction and effect of other cation were optimized. The preconcentration factor is 500 in the optimum analysis situations of whole parameters. Analysis parameters contain the limit of detection and the most capacity of Cobalt (II) ions sorption was achieved 7.0 μg/L and 468 ± 4 µg, respectively. Finally, the Co2+ ions were extracted in various aqueous solutions by this technique.