In this work, the effect of various effective dimensionless numbers and moisture contents on initiation of instability in combustion of moisty organic dust is calculated. To have reliable model, effect of thermal radiation is taken into account. One-dimensional flame structure is divided into three zones: preheat zone, reaction zone and post-flame zone. To investigate pulsating characteristics of flame, goving equations are rewritten in dimensionless space-time (ξ,η,τ) coordinates. By solving these newly achieved goving equations and combining them, which is completely discussed in body of article, a new expression is obtained. By solving this equation, it is possible to predict initiation of instability in organic dust flame. According to the obtained results by increasing Lewis number, threshold of instability happens sooner. On the other hand, pulsating is postponed by increasing Damk?hler number, pyrolysis temperature or moisture content. Also, by considering thermal radiation effect, bing velocity predicted by our model is closer to experimental results.