Because of the simplicity of taking complement operation on alternating (tree) automata and the equivalence relationship between alternating (tree) automata and nondeterministic (tree) automata, the study on alternating (tree) automata becomes a new research area of automata and model checking. Based on notions of L-valued alternating automata and alternating tree automata, the notion of L-valued alternating tree automata is introduce, and closure properties and expressive power of L-valued alternating tree automata are studied. Firstly, it is proved that after taking dual operations on transitions and changing the weight of each final state to its complement, a new L-valued alternating tree automaton is achieved which is the complement of the starting one. Afterwards, the closure is illustrated under conjunction and disjunction of languages accepted by L-valued alternating tree automata. Finally, the expressive power of L-valued alternating tree automata, L-valued tree automata, and L-valued nondeterministic automata are discussed. The equivalence relationship is proved between L-valued alternating tree automata and L-valued tree automata, the algorithms are given between them and complexities are discussed of algorithms; simultaneously, a method is provided to show how to use L-valued nondeterministic automata to simulate L-valued alternating tree automata.