We studied the effect of an Al underlayer on the growth of carbon nanotubes (CNTs) and their field emission characteristics, First of all, CNTs were grown on the Invar catalyst layers with different thickness of 1 to 10 nm, showing that the CNT length was saturated for the catalyst 5 nm or thicker. The CNTs grown on the 5-nm-thick catalyst were ${sim}10{mu}m$ long and ${sim}30nm$ in diameter. Second, an Al underlayer was applied between the catalyst layer and the Ti diffusion barrier to reduce the diameters of CNTs for better field emission properties by forming spherical Al oxide particles on which smaller catalyst nanoparticles would occur. The optimal thickness of an Al underlayer underneath the 5-nm-thick catalyst was ${sim}15nm$, producing the CNTs with the length of ${sim}15{mu}m$ and the diameter of ${sim}15nm$. The field emission measurements, following the tape activation, showed that the thinner and longer CNTs gave rise to better field emission performance with the lower turn-on and threshold electric fields.