The effect of encapsulation of C60 fullerenes upon the buckling strength of hosting carbon nanotubes (CNTs) has been investigated,using molecular dynamics (MD) simulations. The simulation results show that encapsulatingC60 fullerenes into some CNTs with diameters larger than that of the (10, 10) CNT, in particularthe (14, 14) and (18, 18) CNTs, significantly reduces the buckling strength, in contrast tothe conventional wisdom that fillings increase the mechanical strength of hollow structures.The simulations have also confirmed the previous findings that filling a (10, 10) CNT withC60 increases the buckling strength. Our detailed analysis reveals that the interaction between theC60 fullerenes and the hosting (10, 10) CNT is cylindrically symmetric, while the presence of a zigzag arrayof C60 inside the (14, 14) CNT breaks the cylindrical symmetry and so does the presence of the three arraysof C60 inside the (18, 18) CNT. The induced asymmetries cause one peak for theC60@(14, 14) system and threepeaks for the C60@(18, 18) system in the corresponding force distribution along the circumferential direction. The forceconcentration leads to observed reduction in the buckling strength. The reduction is more severe forC60@(14, 14) thanfor C60@(18, 18), because the force distribution of the former system is more asymmetric.