An ab initio calculation is carried out to investigate the optical origin of highly-efficient and selective trans <----> cis isomerizations in a short polyene. It is found that the highly-efficient isomerization is closely related to the highly-selective optical transitions. These dipole-allowed transitions occur selectively at two configurations with the dihedral angles theta(11,12)=60 degrees and 120 degrees, where the system can be excited or de-excited and is also able to undergo cis or trans isomerization effectively. For many configurations which are not favorable to the isomerization, the transition matrix elements are basically zero. This rules out many unfavorable transitions at these configurations, which is why photoinduced isomerization is so efficient.