Abstract The superconducting transition temperatures, Tc, and their pressure dependences ?Tc/?P of some C-15 Laves phase compounds, (Zr0.5Hf0.5-xTa)V2 (x≤0.2) and (Hf0.5Zr0.5-xNbx)V2(x≤0.2) system, have been studied. A description of the experimental apparatus and results is given. Unlike the cases of (Hf1-xTax)V2 and (Zr1-xTax)V2, introduction of Ta in (Zr0.5Hf0.5-xTax)V2 depresses Tc but causes a remarkable higher ?Tc/?P. So the superconducting transition temperature Tc of Zr0.5Hf0.45Ta0.05V2 is higher than that of (Zr0.5Hf0.5)V2 at high pressure. In view of the band theory method with angular momentum representation, a new relation for the pressure dependence of elect-ron-phonon coupling (i.e. ?lnη/?lnV) is derived. It emphasized the relative importance of high angular-momentum-partials variation of the conduction band electron wave function on ?lnη/?lnV. This formulation is helpful in understanding the above-mentioned experimental results and also gives a resonable explanation to the facts that Tc of some elements, e. g. Cs, Ba and La, are substantially enhanced by pressure.