Undivided nave and chancel with gallery to W end. Six-bay roof to nave, partly restored with new wood during the 1974 restoration. The 2 bays over the gallery were altered in the C17-18 and are divided by a collar-beam truss. C15 arch-braced roof over the remaining 4 bays with diagonal struts above the braces. Cusped windbraces, except to W bay; 2nd brace from E has a dentilled soffit. Two rows of purlins, wooden wall-plate. Flagstone floor. The furnishings are mainly C18, with some panelling reused from the C17: Irregular box pews to N side, some with fine Jacobean decoration, that to centre reading 'I. H. 1711' and relating to the Hughes family of Gwerclas. Open wooden benches to S side, in between which is a wood-panelled triple combination of pulpit, reading desk and clerk's desk, also partly reused from former furnishings. Font set into wall to L of porch, a tall bowl, said to be late medieval. Large W gallery reached by spiral stone steps in NW corner. Seating on simple wooden benches of C18 date, and also an enclosure. These benches were mainly for the choir and there is a fine pyramidal music stand. The wood panelled gallery front has a moulded handrail and is plastered to the outer side. Underneath the gallery is a cross-beam with medium chamfer, supported to the L by a tapering stone pier possibly the base of a preaching cross.
The chancel is ceiled by a C15 canopy of honour, though only a small part of the original survives. Fine C18 wooden altar rail with turned balusters; wood panelled reredos. The panelling continues around SE angle of chancel forming a small boxed enclosure, the door reading 'The Rector 1841'. Bench of 1768 to L of altar. To its L, on N wall, is a small pointed-arched door to a cupboard, inside which is a narrow blocked light, said to have been a squint. The box pews continue from the nave into the chancel; one has an arched head to the rear and reads 'W. J. Cefn Cymer, 1759.' The E window has a small area of stained glass to the top, in the form of flowers. It is flanked by Welsh decalogue boards.
The interior has a particularly fine series of wall paintings, undertaken in at least 8 overlapping phases. The N wall includes a depiction of a bishop with hagiography to the rear, possibly representing Jerusalem; this is C14 and Flemish-influenced. Scenes to the L are probably C15; one figure is thought to be St Christopher. To the L of the porch on the S side is another C14 scene including a human head. A wild beast depicting one of the Seven Deadly Sins is to the R of the pulpit, above which are letter inscriptions of c1600. Two further beasts can also be made out. One of the clearest paintings is on the N wall adjacent to the gallery front; a figure of Death of C18 date. The plastered gallery front is decorated with classical-style roundels with a blind arch between and biblical references. These are also C18 and were painted by George Pritchard.