Harmfulness of various microfines in manufactured sand was studied by investigating effects of microfines on workability, compressive strength and drying shrinkage of cement mortars. A modified methylene blue（MB） value based on microfine characteristic and content was proposed. Meanwhile, feasibility of employing the index of modified MB value to evaluate microfine harmfulness was also analyzed. The results indicate that harmfulness of stone powder is quite limited though its content is relatively high, while that of clay is considerable though clay content is low, which confirms that harmfulness of microfines is not only related with microfine content but depended more on microfine mineralogy; strong correlation is found between modified MB value and workability, drying shrinkage and compressive strength of cement mortar, so it can be applied in evaluating microfine harmfulness.