Wavelength conversion is one of the key techniques to improve the blocking performance in wavelength-routed WDM networks. Given that wavelength converters nowadays remain very expensive, how to make effective use of wavelength converters becomes an important issue. In this letter, we analyze the sparse-partial wavelength conversion network architecture and demonstrate that it can significantly save the number of wavelength converters, yet achieving excellent blocking performance. Theoretical and simulation results indicate that, the performance of a wavelength-routed WDM network with only 1-5% off wavelength conversion capability is very close to that with full-complete wavelength conversion capability.