In this paper, we presented a modeling approach to investigate the dynamics of Giardiasis in humans and domestic animals coupled with a contaminated environment. We computed the basic reproduction number R0 and employed it in analyzing the effect of initial transmission and the stability of disease when an outbreak occurs. Results show that even when η= 0, R0= R0h is greater than 4, showing that person-to-person transmission is the most significant in the dynamics of Giardiasis. An increase in η increases the value of R0 to some extent. Numerical simulations show that whenever there is an outbreak of Giardiasis in humans and domestic animals, the disease is likely to persist in the first two months and thereafter it will start to slow down to disease-free-equilibrium.