Water breakthrough is a common problem when an oil field is waterflooded. Polymer solutions are widely applied to resolve the water channeling problem in deep formations. In practice, polymers are used in deep formation treatments either to block in-depth high permeability zones or to improve the mobility ratio. However, the major challenge for in-depth profile modification is how to deliver the gelling solution to the target zones. To this end, a polymer solution has to be injected at a high concentration. Thus, such a treatment sometimes becomes uneconomical. On the other hand, polymer flooding is ineffective primarily due to the viscosity loss caused by mechanical and chemical degradation. In this paper, a weak gel system is developed and used as an in-depth profile modification agent and as an oil displacement agent in a heterogeneous reservoir. The gel system has relatively low strength but can still be crosslinked in the reservoir formation. In comparison with a pure polymer solution, it suffers from less mechanical and chemical degradation because of its crosslink reaction. The EOR mechanisms of the weak gel system are described briefly. The detailed effects of polymer concentration, crosslinker concentration, temperature, and pH on gelation time are studied. Also the theological properties, gelation range, and gel strength are examined. The coreflood results provide experimental evidence that the weak gel acts both as an in-depth profile modification agent and as an oil displacement agent. Furthermore, pilot tests have been performed in the Gudong and the Gudao oil fields of the Shengli oil field in China since 1992. The cummulative incremental oil production has been over 58,000 tonnes at the end of 2001.