We have investigated effects of applied uniaxial pressure (p) on magnetic phase transitions in a triangular lattice antiferromagnet CuFeO2 (CFO) and a slightly Ga-substituted compound CuFe1-xGaxO2 (CFGO) with x = 0.018. We have performed neutron diffraction, spherical neutron polarimetry, magnetic susceptibility and pyroelectric measurements under p up to 100 MPa applied on the [1 (1) over bar0] surfaces of the single crystal samples. As a result, we have revealed that in both of CFO and CFGO (x = 0.018), the application of p significantly increases the transition temperature from the paramagnetic phase to a collinear incommensurate antiferromagnetic phase, at which these systems exhibit a crystal structural distortion associated with the magnetic ordering. This suggests that the application of p beaks equilateral symmetry of the triangular lattice above the original transition temperature, and partially relieves geometrical spin frustration, which generally suppresses long-range magnetic orderings even at low temperatures. In CFGO (x = 0.018), we have also found that the ferroelectric helimagnetic phase, which originally shows up as an intermediate phase in zero pressure, partly remains down to low temperatures under applied p. These results have demonstrated that uniaxial pressure can be an effective tool to control magnetic phase transitions in frustrated magnets.