• Abstract • Background: The present study aimed to assess the therapeutic effect of chitosan nanoparticles and metronidazole against Giardia lamblia as well as evaluate the efficacy of loading metronidazole on chitosan nanoparticles. • Methods: This study was carried out at medical Parasitology Department, Faculty of Medicine, Zagazig University and Theodor Bilharz Research institute(TBRI) from Febreuary 2019 to Febreuary 2020 on forty-five hamsters that were divided into 5 groups 9 hamsters each: Group A non-infected hamsters, Group B infected control group, Group C, D and E infected with G. lamblia and treated with Chitosan nanoparticles (CsNPs), metronidazole (MTZ) and metronidazole-loaded chitosan nanoparticles (MTZ-CsNPs) respectively. • Results: The highest percentage of reduction in the Giardia cyst and trophozoite counts were in group that received MTZ-CsNPs (94.69%, 94.29%). Lower percentages of reduction were recorded for MTZ treated group (90.15%, 89.52%) and CsNPs treated group (63.64%, 75.24%). Histopathological examination showed marked healing of intestinal mucosa after treatment with MTZ-CsNPs. • Conclusion: CsNPs showed a therapeutic effect against Giardia infection in hamsters. Loading of metronidazole on chitosan nanoparticles enhanced therapeutic effect of both CsNPs as well as metronidazole.