3-Hexulose-6-phosphate synthase (HPS) and 6-phosphate-3-hexuloisomerase (PHI) are two key enzymes in the formaldehyde (HCHO) assimilation pathway in methylotrophs. The HPS/PHI fusion protein, encoded by the chimeric gene of hps and phi from Mycobacterium gastri MB19, possesses both HPS and PHI activities in an Escherichia coli transformant. Overexpression of the fusion protein in chloroplasts of geranium (Pelargonium sp. Frensham) created a photosynthetic HCHO assimilation pathway according to C-13-NMR analysis. The transgenic plants exhibited an enhanced ability in HCHO-uptake and [C-14]HCHO-assimilation. Moreover, the transgenic plants showed greater HCHO-resistance and stronger capacity in purification of the HCHO-polluted air. Therefore, the use of the single chimeric gene may not only greatly simplify the transformation procedure but also improve the efficiency of phytoremediating HCHO in ornamental plants.