Tailoring of electromagnetic (EM) parameters has become increasingly important for EM waves propagation, absorption and shielding applications. Single-phase La0.7Sr0.3Mn1-yCoyO3 (LSMCO, y = 0, 0.3, 0.5, and 1.0) have been prepared by a sol-gel process followed by cold pressing and sintering. The effect of Co doping on the electromagnetic properties of LSMO has been investigated. Plasma-like negative permittivity (epsilon r ' < 0) and negative susceptibility (mu r ' appear simultaneously in La0.7Sr0.3MnO3 (y = 0, LSMO). The negative permittivity for frequencies ranging from 0.01 to 1 GHz results from plasma oscillation of conduction electrons in LSMO, and the frequency dispersion agrees well with the Drude model. A higher positive permittivity was also obtained with increasing Co content, indicating a wide adjustment range of permittivity from - 1.3 x 10(4) to 2.9 x 10(4), which represents an important step toward the development of new capacitors with a higher capacitance for integrated circuit. Furthermore, tunable negative susceptibility is observed for LSMO and La0.7Sr0.3CoO3 (y = 1.0, LSCO) samples over frequency ranges of 0.1-1 GHz and 0.01-1 GHz, respectively. These results open the door for the realization of tunable negative electromagnetic properties in single-phase perovskites, which is of great significance for the development of negative parameters materials.