Primary purpose of this lecture is to presentation on Transformer Regulation. Regulations of a transformer means the difference between the full-load and no-load supplementary terminal voltages expressed as a percentage of the full-load voltage. Here also discuss on efficiency of a transformer, it is a power factor means the output divided by the input- the two being measured from the same unit. This lecture also explain Condition for Maximum Efficiency, Uses of Auto-Transformer, Additive Polarity and Subtracting Polarity. Finally discuss on Parallel Operation of Single-Phase Transformer and Three Phase Transformer with mathematical examples and graph.