β oscillations in local field potentials, electrocorticography (ECoG), and electroencephalograms (EEG) are ubiquitous in the motor cortex of monkeys and humans. However, relations between their dynamical properties and behavior have not been well studied. Here, we used ECoG grids to cover large areas of motor cortex and some somatosensory cortex in monkeys while they performed an unconstrained reaching and a lever pulling task in three dimensional space with or without go cue. We used percentage of phase locking (PPL) as a dynamical property of β oscillations aligned to behaviorally relevant events, reaching onsets (RO), lever onset (LO), and holding onset (HO) as well as multiple peaks of electromyography (EMG) recorded from various upper limb muscles. We showed that relative strength of PPL alinged to RO and LO were reserved with or without the external go cues. Furthermore, among the muscles that we recorded EMGs from, β oscillations were not closely phase locked to any of EMG onsets. Therefore, phase locking of β oscillations is related more to the attentive state and external cues as opposed to detailed muscle activities.