The electron capture decay rate of Be-7 encapsulated in carbon nanotubes (CNTs) is studied by using a first-principles approach. We calculate the electron density at the Be nucleus position, which is proportional to the magnitude of the decay rate. With decreasing the radius of the CNT, a drastic decrease following a slight increase in the electron density is observed. The magnitude of the largest density in these complex nanostructures is nearly equal to the maximum value that has been reported previously in Be-7@C-60 and 1.64% larger than that of the density at the Be nucleus embedded in the Be metal. (C) 2013 Published by Elsevier B.V.