Nowadays boron compounds are widely used for wood preservation. They have several good properties but some weaknesses, too. One of the most important benefits is their efficacy against wood pests, and their weakness insufficient fixation in wood. Despite of the fact, that they are applied for almost 100 years in the field of wood preservation, in literature there are several controversial data about their efficacy. Therefore, minimal inhibitory concentration in nutrient medium and wood blocks were determined. For testing, 5 brown rot (Antrodia vaillanti, Gloeophyllum trabeum, Coniophora puteana, Lentinus lepideus and Serpula lacrymans), 2 white rot (Trametes versicolor and Hypoxylon fragiforme) and 2 blue stain fungi (Aureobasidium pullulans and Sclerophoma pithyophila) were applied. Results of the fungicidal testing of boric acid confirmed that boric acid is a very effective fungicide inhibiting fungal growth at relatively low concentrations. The most boron sensitive fungus was Coniophora puteana, and the most resistant one Hypoxylon fragiforme.