Various physical properties of pseudobinary (1-x) $Pb(Fe\frac{1}{2}Nb\frac{1}{2})O_3$-x $Pb(Co\frac{1}{2}W\frac{1}{2})O_3$ system has been investigated by the X-ray diffraction, dielectric constant, and differential scanning calorimetric (DSC) measurements. Specimens are prepared by the conventional mixed oxide method. X-ray diffraction at room temperature show that the sintered ceramic compact has formed a perfect solid solution of a perovskite structure through the whole composition range. The behavior of dielectric constant as a function of temperature may be categorized into two distinct classes. Firstly in the mid-composition range of 0.2≤x≤0.8, the dielectric constant shows a diffused maximum, and the apparent transition temperature corresponding to the dielectric constant maximum is shifted to a higher temperature with the increase of the external applied frequency. Secondly, for x≥0.9, and x≤0.1, the dielectric constant maximum is relatively sharp, and no dielectric dispersion of the dielectric constant peak temperature is observed. The phase diagram of the present system is concave upward. Anomalous specific heat curves are obtained from in the antiferroelectric $Pb(Co\frac{1}{2}W\frac{1}{2})O_3-$ rich region. Especially pure $Pb(Co\frac{1}{2}W\frac{1}{2})O_3$ shows two anomalous maxima in the specific heat curve at 36℃ and -17℃. The anomaly around 36℃ seems to be associated with the antiferroelectric-paraelectric phase transition, whereas the anomaly around -17℃ is reckoned to be the first order transition from the structural change coupled with the volumetric change.