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We present the analysis of Blazhko-type modulation in double-mode RR Lyrae(RRd) stars found in the OGLE photometry of the Galactic bulge. Modulation isdetected in 15 stars. Most of them have non-typical period ratio of the radialmodes. In the Petersen diagram, at a given period of the fundamental mode, theyare located significantly below or above the sequence formed by the majority ofRRd stars.  Multi-periodic modulation is very frequent; two or three modulation periodsare detected in 8 stars. Modulation periods vary from ~20 to more than 300days. Radial mode amplitudes can be modulated by a few to nearly hundred percent. Both radial modes may be modulated with the same period. More commonlyhowever, dominant modulation for the fundamental mode has different period thandominant modulation for the first overtone. Quite often modulation of only onemode is detected in the data. We find a clear feedback between pulsationamplitude of the dominant mode and mean stellar brightness: lower the pulsationamplitude, brighter the star. At phases of low pulsation amplitude, the modeperiods are prone to fast changes. All the stars share the common feature:their pulsation properties are non-stationary. Amplitudes and phases of theradial modes vary irregularly on a long time-scale of a few hundred or thousanddays. The short-term modulations are also irregular.  One of the stars has switched the pulsation mode recently: from single-modefundamental mode pulsation to RRd state. In other star the non-radial mode withcharacteristic ~0.61 period ratio to the first overtone is detected. Thisnon-radial mode is likely modulated with the same period as the radial modes.

Blazhko-type modulation in the double-mode RR Lyrae stars of the OGLE Galactic bulge collection