Ionization potential of excited states have been calculated for the 1s(2)2s(2)2p(2) P-3(2), 1s(2)2s(2)2p3s 3P(2)(0), 1s(2)2s(2)2p3d D-3(3)0, 1s(2)2s(2)2p3d F-1(3)0, and 1s(2)2s(2)2p4d D-3(0)3 series of carbon-like sequence from Z = 6 to Z = 23 on the basis of the weakest bound electron potential model theory and the concept of iso-spectrum-level series. In the calculation, the non-relativistic weakest bound electron potential model theory is used to calculate the non-relativistic energies and a fourth-order polynomial in nuclear charge Z is employed to evaluate the relativistic correction. Calculated values are compared with the experimental compiled results. Agreement with the experimental compiled results is generally within 0.1%. (C) 2003 Elsevier B.V. All rights reserved.