An improved structure of transformer's iron core mainly has an iron core main body, having a salient main core part with a rectangular horizontal cross-section, furnished at the center part thereof. The main core part has a pair of oppositely configured side wing parts furnished at the outside thereof. A containing circumferential channel formed between the main core part and the side wing parts has a pair of oppositely configured side openings making the containing circumferential channel form communicative exits toward the outside through the two side openings. By employing the salient main core part with a rectangular-shaped horizontal cross-section, the invention is capable of forming longer outer circumference to effectively increase the area of the magnetic flux thereof under the premise of same bobbin. Moreover, in association with the relatively larger side openings that are capable of providing lead frame containing in the containing ring-trench to be outwardly connected to a relatively wider wire connecting seats, the invention is capable of satisfying the required specification of multiple wire-drawing.