Organic Rankine cycle system (10, 110, 210) with cascade cycles comprising a first organic Rankine cycle (20, 120, 220) which operates at high temperature, in which a first organic working fluid carries out a heat exchange with a hot source fluid and at least a second organic Rankine cycle (30, 130, 230, 250) which operates at a temperature lower than the temperature of the first organic Rankine cycle (20, 120, 220) and in which a second organic working fluid carries out a heat exchange with the same hot source, each organic Rankine cycle (20, 120, 220, 30, 130, 230, 250) comprising: - at least a feed pump (24, 124, 224, 34, 134, 234, 254) to feed the organic working fluid in the liquid phase, - at least a first preheater (25, 125, 35, 135, 255) for preheating the organic working fluid, - at least an evaporator (21, 121, 221, 31, 131, 231, 251) to vaporize the organic working fluid, - at least an expansion turbine (22, 122, 222, 32, 132, 232, 252) which expands the vapor of the organic working fluid, - at least a condenser (23, 123, 223, 33, 133, 233, 253) which brings the organic working fluid back into the liquid phase, wherein the evaporator (21, 121, 221) of the first organic Rankine cycle (20, 120, 220) is fed by the entire flow rate (40, 140, 240) of the hot source fluid, said organic Rankine cycle system (10, 110, 210) being characterized in that - the evaporator (31, 131, 231) and the preheater (35, 135, 235) of said at least a second organic Rankine cycle (30, 130, 230) are fed by a first partial flow (43, 143, 243) of the hot source fluid, and - a remaining second partial flow (42, 142, 242) of the hot source fluid is used to at least partially carry out the preheating of the organic working fluid of the first organic Rankine cycle (20, 120, 220). - at least a feed pump (24, 124, 224, 34, 134, 234, 254) to feed the organic working fluid in the liquid phase, - at least a first preheater (25, 125, 35, 135, 255) for preheating the organic working fluid, - at least an evaporator (21, 121, 221, 31, 131, 231, 251) to vaporize the organic working fluid, - at least an expansion turbine (22, 122, 222, 32, 132, 232, 252) which expands the vapor of the organic working fluid, - at least a condenser (23, 123, 223, 33, 133, 233, 253) which brings the organic working fluid back into the liquid phase, wherein the evaporator (21, 121, 221) of the first organic Rankine cycle (20, 120, 220) is fed by the entire flow rate (40, 140, 240) of the hot source fluid, said organic Rankine cycle system (10, 110, 210) being characterized in that wherein the evaporator (21, 121, 221) of the first organic Rankine cycle (20, 120, 220) is fed by the entire flow rate (40, 140, 240) of the hot source fluid, said organic Rankine cycle system (10, 110, 210) being characterized in that - the evaporator (31, 131, 231) and the preheater (35, 135, 235) of said at least a second organic Rankine cycle (30, 130, 230) are fed by a first partial flow (43, 143, 243) of the hot source fluid, and - a remaining second partial flow (42, 142, 242) of the hot source fluid is used to at least partially carry out the preheating of the organic working fluid of the first organic Rankine cycle (20, 120, 220).