Partially Observable Markov Decision Process (POMDP) is a general sequential decision-making model where the effects of actions are nondeterministic and only partial information about world states is available. However, finding near optimal solutions for POMDPs is computationally difficult. Value iteration is a standard algorithm for solving POMDPs. It conducts a sequence of dynamic programming (DP) updates to improve value functions. Value iteration is inefficient for two reasons. First, a DP update is expensive due to the need of accounting for all belief states in a continuous belief space. Second, value iteration needs to conduct a large number of DP updates before its convergence. This thesis investigates two ways to accelerate value iteration. The work presented centers around the idea of conducting DP updates and therefore value iteration over a belief subspace, a subset of belief space. The first use of belief subspace is to reduce the number of DP updates for value iteration to converge. We design a computationally cheap procedure considering a belief subspace which consists of a finite number of belief states. It is used as an additional step for improving value functions. Due to additional improvements by the procedure, value iteration conducts fewer DP updates and therefore is more efficient. The second use of belief subspace is to reduce the complexity of DP updates. We establish a framework on how to carry out value iteration over a belief sub-space determined by a POMDP model. Whether the belief subspace is smaller than the belief space is model-dependent. If this is true for a POMDP, value iteration over the belief subspace is expected to be more efficient. Based on this framework, we study three POMDP classes with special problem characteristics and propose different value iteration algorithms for them. (1) An informative POMDP assumes that an agent always has a good idea about the world states. The subspace determined by the model is much smaller than the belief space. Value iteration over the ...