Although the ventral and dorsal regions of the lateral prefrontal cortex (lPFC) are anatomically distinct, their functional differentiation is still controversial. Local field potentials (LFPs) reflect synaptic input and are widely modulated by information from both the external world and the internal state of the brain. However, functional mapping using LFPs has not been fully tested and is expected to provide new insights into their differences. Thus, the present study analyzed the task-phase-dependent modulations of LFPs recorded from the lPFC of monkeys as they performed a shape manipulation task. Hierarchical cluster analyses of the LFP time-frequency spectra revealed characteristic patterns, especially in the theta and low gamma ranges. In particular, the theta range distinguished the ventral and dorsal parts of the lPFC well. The ventral part exhibited a block of similar LFP patterns whereas the dorsal part showed scattered patterns of small or single sites with different LFP patterns. These results suggest that functional segregation within the lPFC, especially between the ventral and dorsal regions, can be evaluated using task-phase-dependent time-frequency modulations in LFPs.