Measurements of the transition temperature Tc, the second critical filed Hc2 and the magnetic penetration depth λ under hydrostatic pressure (up to 9.2 kbar) in the YB6 superconductor were carried out. A pronounced and negative pressure effects (PE) on Tc and Hc2 with dTc/dp = −0.0547(4) K/kbar and µ0dHc2(0)/dp = −4.84(20) mT/kbar, and zero PE on λ(0) were observed. The PE on the coherence length dξ(0)/dp = 0.28(2) nm/kbar was calculated from the measured pressure dependence of Hc2(0). Together with the zero PE on the magnetic penetration depth λ(0), our results imply that the Ginzburg-Landau parameter κ(0) = ξ(0)/λ(0) depends on pressure and that pressure "softens" YB6, e.g. moves it to the type-I direction.PACS numbers: 74.70. Dd, 74.62.Fj, 74.25.Ha The Ginzburg-Landau parameter κ = λ/ξ (λ is the magnetic penetration depth and ξ is the coherence length) is one of the fundamental parameters for superconductors. The parameter κ establishes the border between type-I (κ < 1/ √ 2) and type-II (κ > 1/ √ 2) superconductors by determining point where the surface energy, associated with the domain wall between the superconducting and the normal state areas, changes their sign from "plus" to "minus". Remarkably, two physical quantities entering κ (λ and ξ) depend on different properties of the superconducting material. Indeed, for BCS superconductors the zero temperature coherence length obeys the relation [1]Here, v F is the average Fermi velocity and ∆(0) is the zero temperature value of the superconducting energy gap. The zero-temperature penetration depth is given by [2,3] 1where the integral runs over the Fermi surface S F . Assuming spherical Fermi surface Eq. (2) reduces toA quick glance to Eqs. (1) and (3) Experiments under pressure open a possibility to probe this phenomena. The reason for this can be understood by considering the simple metal superconductors, like Sn, In, Pb, Al, etc., where the conduction electrons possess s, or p character. Since the s and p electrons in simple metals are nearly free, one expects approximately v F ∝ S F ∝ V +2/3 (V is the unit cell volume). For typical values of the bulk modulus B = 400−800 kbar [5] it leads to d ln v F /dp = d ln S F /dp = 2/3 · B −1 =0.08-0.17 %/kbar. Note that this effect is almost one order of magnitude smaller than the corresponding pressure effect on T c : Sn (d ln T c /dp = −1.30 %/kbar), In (-1.12 %/kbar), Pb (-0.51 %/kbar), Al (-2.47 %/kbar) [4]. Taking into account this and the fact that within BCS theory ∆(0) is simply proportional to T c (2∆(0)/k B T c = 3.52, see e.g. [1]), Eq. (1) implies that the pressure shift of the coherence length ξ(0) is almost the same as the one of T c d ln ξ(0) dpFrom the other hand, the pressure effect on λ(0) [see Eq. (3)] is determined by the pressure induced change of the Fermi surface and the Fermi velocity d ln λ(0) dpwhich appears to be an order of magnitude smaller as shown above. Thus, the much bigger pressure effect (PE) on ξ(0) in comparison with the one on λ(0) implies pressure dependence of the Ginzbur...