This paper propeses a solution for optimization of rice field irrigation channel using generalized minimal spanning tree. GMST is a minimum spanning tree wich concist of several clusters, where the minimum inter-cluster connected by a node. rice field irrigation channel is an example of clustering graph, which rice field is partitioned into many parts. at rice field irrigation channer, one cluster made of a few nodes and one node can be belongs to many cluster, the node is the corner of rice field.rice field irrigation channel optimazattion is a special case of GMST, because there must be one water source and one node be can be at many cluster. to solve the special case, we designed modified prim algorithm and genetic algorithm. the result of this research is minimum sum of rice field irrigation channel and minimum total lenght of rice field irrigation channel, which guaranteed the water can distribution to all rice fields. at the end, it expected to decrease cost and energy for buildingg water irrigation channel.