We show how to set up a forward rate model in the presence of volatility uncertainty

by using the theory of G-Brownian motion. In order to formulate the model, we extend

the G-framework to integration with respect to two integrators and prove a version of

Fubini's theorem for stochastic integrals. The evolution of the forward rate in the model

is described by a diffusion process, which is driven by a G-Brownian motion. Within

this framework, we derive a sufficient condition for the absence of arbitrage, known as

the drift condition. In contrast to the traditional model, the drift condition consists of

two equations and two market prices of risk, respectively, uncertainty. Furthermore, we

examine the connection to short rate models and discuss some examples.