Oseltamivir is a potent neuraminidase inhibitor for influenza treatment. By replacing the carboxylate group in oseltamivir with phosphonate monoalkyl ester, a series of tamiphosphor derivatives were synthesized and shown to exhibit high inhibitory activities against influenza viruses. Our molecular modeling experiments revealed that influenza virus neuraminidase contains a 371-cavity near the S1-site to accommodate the alkyl substituents of tamiphosphor monoesters to render appreciable hydrophobic interactions for enhanced affinity. Furthermore, guanidino-tamiphosphor (TPG) monoesters are active to the oseltamivir-resistant mutant. TPG monohexyl ester 4e having a more lipophilic alkyl substituent showed better cell permeability and intestinal absorption than the corresponding monoethyl ester 4c, but both compounds showed similar bioavailability. Intranasal administration of TPG monoesters at low dose greatly improved the survival rate of mice infected with lethal dose of H1N1 influenza virus, whereas 4c provided better protection of the infected mice than oseltamivir and other phosphonate congeners by oral administration.