We investigate whether pc-scale jets of 3C 345 and 3C 279 are dominated by a normal plasma or an electron-positron plasma. We present a general condition that a jet component becomes optically thick for synchrotron self-absorption. The general condition gives a lower limit of the electron number density, with the aid of the surface brightness condition, which enables us to compute the magnetic field density. Comparing the lower limit with another independent constraint that is deduced from the kinetic luminosity, we can distinguish the matter content. We apply the procedure to the five components of 3C 345 (C2, C3, C4, C5, and C7) and the three components of 3C 279 (core, C3 and C4) of which angular diameters and radio fluxes at the peak frequencies were obtainable from literature. Evaluating the Doppler factors by their equipartition values, we find that four of the five components of 3C 345 and all the three components of 3C 279 are likely dominated by an electron-positron plasma.