Invasive Weed Optimization (IWO) algorithm is inspired by biological growth of weed plants. It is a new derivative free numerical stochastic algorithm based on population. Colonizing behaviour of weed plants is utilized in this algorithm. On survival of fittest theory, more fit plants produce more seeds as compared to less fit plants. But since, there may be a probability that some of the less fit plants may have more important information than the higher fit plants, so in weed colony all plants are given chance to participate in process of reproduction. If plants with lesser fitness produce seeds of high fitness they can survive. This property is utilized as novelty of IWO algorithm.