The investigation of the recently discovered high-Tc superconductors based on iron pnictides, has attracted great attention. Much effort has been devoted to determining the symmetry and structure of the superconducting gap, which is one of the keys to understanding the microscopic pairing mechanism giving rise to superconductivity. However, even for the most studied 122 system, the situation is still fairly complex. Recently, an ultra-low-temperature heat transport study gave strong evidence for nodal superconductivity in optimally doped Ba(Fe${0.64}$Ru${0.36}$)$_{2}$As$_2$. It is interesting to check the superconducting gap structure in this sample by another technique. We will investigate the magnetic penetration depth $\lambda$ by transverse-field $\mu$SR. $\lambda$ is related to the superfluid density, whose temperature dependence is directly determined by the gap function.