We study the weak convergence of the family of processes {V n (t)} n∈N defined bywhere {θ n (u)} n∈N is a family of processes converging in law to a Brownian motion, as n → ∞. We consider two cases of {θ n }. First, we construct θ n based on the wellknown Donsker's theorem and show that {V n (t)} n∈N converges in law to a multifractional Brownian motion of Riemann-Liouville type, as n → ∞. Second, we construct θ n based on a Poisson process, and then show that a multifractional Brownian motion of Riemann-Liouville type can be approximated in law by {V n (t)} n∈N .