The experimental results of the effect of the magnetic field (up to 60kOe) on the broadening of the resistive transition of bulk composites Y3/4Lu1/4Ba2Cu3O7 (YBCO)+CuO are presented. These composites represent the network of the tunnel-type Josephson junctions where the copper oxide acts as a material forming barriers between YBCO crystallites. The mechanisms responsible for broadening of the resistive transition under magnetic field are discussed. The analysis of experimental R(T) dependences have shown that in the low field range 0–102Oe, the R(T) dependences are described well by the Ambegaokar–Halperin (AH) model. In the range 103–6×104Oe, the dissipation follows Arrhenius law R∼exp(−U(H)/kBT) characteristic for thermally activated flux creep model. In the range H∼102–103, the crossover from AH to flux creep dissipation mechanisms occurs.