  We study the transport properties, in particular, the thermoelectric figure
of merit ZT of armchair graphene nanoribbons, AGNR-N (for N=4-12, with widths
ranging from 3.7 to 13.6~\AA) through strain engineering, where N is the number
of carbon dimer lines across the AGNR width. We find that the tensile strain
applied to AGNR-$N$ changes the transport properties by modifying the
electronic structures and phonon dispersion relations. The tensile strain
increases the ZT value of the AGNR-$N$ families with N=3p and N=3p+2, where $p$
is an integer. Our analysis based on accurate density-functional theory
calculations suggests a possible route to increase the ZT values of AGNR-$N$
for potential thermoelectric applications.
