An equivalent parallel randomized clinical trial was approved by the local University Committee of Ethics. 260 children with necrotic primary molars were allocated into 2 equal groups (82 children per group). The participants were allocated into two groups; group "1" teeth were instrumented with OneShape rotary system (Micro Mega, Besancon, France), and group "2" teeth were instrumented with WaveOne Gold reciprocating system (Dentsply Maillefer, Ballaigues, Switzerland). Postoperative pain was assessed using a four-point pain intensity rating scale adopted from Wong-Baker FACES (WBF) pain scale . The face scale consists of four faces with a brief title describing each face representing a scale from the first face (score 1), second face (score 2), third face (score 3), and forth face (score 4) 3. Postoperative pain was assessed at 6 six time intervals (at 6, 12, 24, 48, 72 hours and one-week).

Inclusion Criteria:

1. Children ages from 4 to 8 years classified as class I or II according to the American Society of Anesthesiologists (ASA).
2. No previous dental experience.
3. Non-vital primary molar (necrotic or with chronic apical periodontitis) which confirmed radiographicly with the presence or absence of periapical radiolucency.
4. Severe tooth mobility.

Exclusion Criteria:

1. Severe behaviour or emotional disabilities.

2. Analgesic intake in the preceding 12 hours.

3. Non-restorable crowns of primary molars.

4. Radiographic presence of internal root resorption or massive bone destruction.

5. Root resorption exceeded one-third of the root length.

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children aged from 4 to 8 years