We have investigated the magnetoelectric (ME) properties of the orthorhombic RMnO3 (R=mixed rare earth ions) crystals with and without the 4f magnetic moments, while keeping an average ionic size of R. In the presence of the magnetic contribution from the R ions ((Eu,Ho)MnO3), we have observed the ferroelectric polarization along the c axis (Pc
) in a zero field, which arises from the bc spiral structure stabilized by the anisotropic exchange interaction between the R and Mn sublattices like TbMnO3. The Pc
phase can be transferred to Pa
by application of fields of ∼1.5 T parallel to the b axis. The observed decrease of transition field compared with TbMnO3 suggests that the magnetic contribution and the local distortion of the R ions are key factors for governing the ME properties.