We propose quantum process tomography (QPT) using an arbitrary number of ancillary qubits as a compromise between the advantages of the original QPT and the QPT using ancilla not smaller than the system. For an n-qubit system with m(< n) qubit ancilla, the number of total qubits and the number of measurements are n + m and 2(2(n-m)), respectively, compared to n and 2(n) in the former QPT and 2n and 1 in the latter QPT. As an example, QPT using a two-qubit operation with one ancillary qubit in nuclear magnetic resonance is studied. The initial states and the pulse sequences to produce them from the thermal equilibrium state are presented.