The dielectric properties, electric modulus and ac electrical conductivity (sigma(ac)) of Al/Co-doped (PVC + TCNQ)/p-Si structures have been investigated in the wide frequency and voltage range of 0.5 kHz-3 MHz and (-4 V)-( 9 V), respectively, using the capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements at room temperature. The real and imaginary parts of dielectric constant (epsilon ', epsilon ''), loss tangent (tan delta), sigma(ac) and the real and imaginary parts of electric modulus (M ', M '') were found strongly function of frequency and applied voltage especially at low frequencies. The epsilon '-V plot shows an anomalous peak in the forward bias region due to the series resistance (R-s), surface states (N-ss) and interfacial layer (PVC + TCNQ) effects for each frequency and then it goes to negative values known as negative dielectric constant (NDC) at low frequencies (f <= 70 kHz). Such observation of NDC is important result because it implies that an increment of bias voltage produces a decrease in the charge on the electrodes. The amount of negativity epsilon ' value increases with decreasing frequency and this decrement in the NDC corresponds to the increment in the epsilon ''.