We studied the electronic transport properties of a new type of iron(II) porphyrin (PP-Fe) chemosensor for CO with two single-walled carbon nanotube (SWCNT) electrodes, using the first-principle density functional theory. Owing to the fact that electron does not pass through the Fe center when the CO molecule is absorbed to PP-Fe, leading to a considerable difference between the I-V curves of PP-Fe before and after CO absorption, it has great potential for application as a chemical sensor for CO.