Flux pinning and the optimization of J(,c) properties have been studied for in situ prepared Nb(,3)Sn-Cu superconducting composite wire. The pinning force density is found to be a universal function of magnetic fields as F(,p) (PROPORTIONAL) h(' 1/2)(1-h)('2) over the full range of reduced field density, h, from h = 0.1 to h = 1.0. The experimental results for a wide range of micro-structures show that J(,c) is controlled by core pinning on surfaces of grain boundaries, and pinning by shear forces in flux line lattice at high fields is not important. The effective pinning area per unit volume calculated from theory is found to be equal to the area of the interfaces of Nb(,3)Sn-Nb(,3)Sn grain boundaries which are perpendicular to the filament axis. An attempt to increase J(,c) properties in these wires is made by eliminating C impurities, optimizing as-drawn Nb filament size and alloying with the third element Ta. J(,c) is increased by a factor of 2 to 5 by reducing C content in the alloy from 380 ppmw to 15 ppmw. Optimum Nb filament thicknesses to produce maximum J(,c) and H(,c2)('*) are found to be 600 (')(ANGSTROM) and 700 (')(ANGSTROM) for 550(DEGREES)C diffusion annealing and 650(DEGREES)C diffusion annealing, respectively. In addition, Ta additions in the alloy lead to an increase of J(,c) at magnetic fields above 12T. It is found that a small amount of Ta in composite wire can raise H(,c2)('*) as well as J(,c);(');('1)This work was done at the Ames Laboratory, Iowa State University, Ames, IA, operated for the U.S. Department of Energy by I.S.U. under contract No. W-7405-eng-82. The research was supported by the Director of Energy Research, Office of Basic Energy Sciences, WPAS-KC-02-01.