Pre-print, Working paper: In urban China, urban resident annual earnings are 1.3 times larger than long term rural migrant earnings as observed in a nationally representative sample in 2002. Using microsimulation, we decompose this difference into four sources, with particular attention to path dependence and statistical distribution of the estimated effects: (1) different allocation to sectors that pay different wages (sectoral effect); (2) hourly wage disparities across the two populations within sectors (wage effect); (3) different working times within sectors (hours effect); (4) different population structures (population effect). Although sector allocation is extremely contrasted, with very few migrants in the public sector and very few urban residents working as self-employed, this has no clear impact on differential earnings. Indeed, the sectoral effect is not robust to the path followed for the decomposition. We show that the migrant population has a comparative advantage in the private sector: increasing its participation into the public sector would not necessarily improve its average earnings. The second main finding is that the population effect is robust and significantly more important than wage or hours effects. This implies that the main source of disparity between the two populations is pre-market (education opportunities) rather than on-market.